<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2025 Workiva-->
<!--r:7a45eeab-ca45-4b53-a62e-10fc39ad530a,g:edce3dca-cd51-4d45-9826-f3507b427af7-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://i3verticals.com/role/COVER" xlink:type="simple" xlink:href="iiiv-20250630.xsd#COVER"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/COVER" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_440f3ca1-1267-4a8a-9c5e-4bf06860b0f9" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_DocumentInformationTable_440f3ca1-1267-4a8a-9c5e-4bf06860b0f9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_5859e529-957d-48c5-a5ed-4391d7a9b4cf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_440f3ca1-1267-4a8a-9c5e-4bf06860b0f9" xlink:to="loc_us-gaap_StatementClassOfStockAxis_5859e529-957d-48c5-a5ed-4391d7a9b4cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_5859e529-957d-48c5-a5ed-4391d7a9b4cf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_5859e529-957d-48c5-a5ed-4391d7a9b4cf" xlink:to="loc_us-gaap_ClassOfStockDomain_5859e529-957d-48c5-a5ed-4391d7a9b4cf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_e63ff24b-c38d-42d5-8e5f-b35a88973849" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_5859e529-957d-48c5-a5ed-4391d7a9b4cf" xlink:to="loc_us-gaap_ClassOfStockDomain_e63ff24b-c38d-42d5-8e5f-b35a88973849" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_60eb5bf7-4318-44fa-af24-4a225a3b40a3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_e63ff24b-c38d-42d5-8e5f-b35a88973849" xlink:to="loc_us-gaap_CommonClassAMember_60eb5bf7-4318-44fa-af24-4a225a3b40a3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_da8962f4-4b73-4c82-b36e-1f1df71d7910" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_e63ff24b-c38d-42d5-8e5f-b35a88973849" xlink:to="loc_us-gaap_CommonClassBMember_da8962f4-4b73-4c82-b36e-1f1df71d7910" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_a0e052bc-1d4f-4aaa-8e10-d45f1782e3cf" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_DocumentType_a0e052bc-1d4f-4aaa-8e10-d45f1782e3cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_708ee20f-6f21-4524-9b8d-77e90c90a827" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_DocumentQuarterlyReport_708ee20f-6f21-4524-9b8d-77e90c90a827" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_e9e77c54-5ba3-4ac9-b1a7-ebf5f931e36c" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_DocumentPeriodEndDate_e9e77c54-5ba3-4ac9-b1a7-ebf5f931e36c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_0f3a7088-c8e5-4404-bdd0-75e5e483dec2" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_DocumentTransitionReport_0f3a7088-c8e5-4404-bdd0-75e5e483dec2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_7d7ea765-dca0-4642-8d36-e964b59a4235" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityFileNumber_7d7ea765-dca0-4642-8d36-e964b59a4235" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_43372947-a80a-4858-ae5e-5f51ee9f5421" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityRegistrantName_43372947-a80a-4858-ae5e-5f51ee9f5421" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_5d36a863-b451-4a4d-b4b0-f3988112b801" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityIncorporationStateCountryCode_5d36a863-b451-4a4d-b4b0-f3988112b801" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_ecba0e8b-d7ab-4868-8c71-9a7a676178df" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityTaxIdentificationNumber_ecba0e8b-d7ab-4868-8c71-9a7a676178df" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_e710c0d1-dc8e-4a90-b7b2-768c7de739ca" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityAddressAddressLine1_e710c0d1-dc8e-4a90-b7b2-768c7de739ca" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine2_d6c00826-ebcb-4c65-a027-1829eeafb654" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressAddressLine2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityAddressAddressLine2_d6c00826-ebcb-4c65-a027-1829eeafb654" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_c00a0357-a662-40ca-b6c8-5d40d85dbbae" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityAddressCityOrTown_c00a0357-a662-40ca-b6c8-5d40d85dbbae" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_1143077f-03db-42b9-908b-55fcd2fb1114" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityAddressStateOrProvince_1143077f-03db-42b9-908b-55fcd2fb1114" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_0c5904db-ab57-4b45-8bd7-ce2e2094d27d" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityAddressPostalZipCode_0c5904db-ab57-4b45-8bd7-ce2e2094d27d" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_7cc0911c-2f6c-447f-beb4-54856171e544" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_CityAreaCode_7cc0911c-2f6c-447f-beb4-54856171e544" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_4715b6fb-3683-46ba-8fc4-0816a3b875fa" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_LocalPhoneNumber_4715b6fb-3683-46ba-8fc4-0816a3b875fa" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_3f0ba17d-21e2-4e0e-8e6e-9224e3d8c4ee" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_Security12bTitle_3f0ba17d-21e2-4e0e-8e6e-9224e3d8c4ee" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_49f86d28-10c0-4275-bb83-c3eb26ea2b56" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_TradingSymbol_49f86d28-10c0-4275-bb83-c3eb26ea2b56" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_ebc2b32f-ec33-4773-bdbe-6455e0a77a6f" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_SecurityExchangeName_ebc2b32f-ec33-4773-bdbe-6455e0a77a6f" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_00f84630-cde7-48e1-9562-89749f1e7b88" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityCurrentReportingStatus_00f84630-cde7-48e1-9562-89749f1e7b88" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_f385f6e7-2289-4d7d-b007-3392dc46a23e" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityInteractiveDataCurrent_f385f6e7-2289-4d7d-b007-3392dc46a23e" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_d2ad2a00-f30e-4464-885b-e27ab3def484" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityFilerCategory_d2ad2a00-f30e-4464-885b-e27ab3def484" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_ea21dba9-5506-4385-90da-d3673b80beb4" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntitySmallBusiness_ea21dba9-5506-4385-90da-d3673b80beb4" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_02cc7057-3bb1-4a2e-ab09-e596b6f79dbb" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityEmergingGrowthCompany_02cc7057-3bb1-4a2e-ab09-e596b6f79dbb" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_fe07e9fc-9943-4d05-9720-170cefb60d86" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityShellCompany_fe07e9fc-9943-4d05-9720-170cefb60d86" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_9102004a-6a24-405d-8964-c3be924842ea" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_9102004a-6a24-405d-8964-c3be924842ea" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_2191be1d-dafb-4866-947a-49bdaedd734e" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_EntityCentralIndexKey_2191be1d-dafb-4866-947a-49bdaedd734e" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_4b03c81a-9bb0-4171-bae8-cbd60652be62" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_CurrentFiscalYearEndDate_4b03c81a-9bb0-4171-bae8-cbd60652be62" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_ef55dd60-a199-4606-8247-8afbdbfcb01a" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_DocumentFiscalYearFocus_ef55dd60-a199-4606-8247-8afbdbfcb01a" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_5a45e4e3-80a4-4b03-ae19-a46a6d3ec971" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_DocumentFiscalPeriodFocus_5a45e4e3-80a4-4b03-ae19-a46a6d3ec971" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_1805757b-3260-4789-9fd2-f389bf698ff0" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_07f84c85-b9c8-438a-9150-34249fac659f" xlink:to="loc_dei_AmendmentFlag_1805757b-3260-4789-9fd2-f389bf698ff0" xlink:type="arc" order="29"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSUNAUDITED" xlink:type="simple" xlink:href="iiiv-20250630.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETSUNAUDITED"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSUNAUDITED" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_42df5dd9-f80c-41b3-8e52-503337a18bdc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_a24ee50b-8da6-4d8d-b6c4-a4804bce6661" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_42df5dd9-f80c-41b3-8e52-503337a18bdc" xlink:to="loc_us-gaap_StatementTable_a24ee50b-8da6-4d8d-b6c4-a4804bce6661" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_acf9394f-663b-41b9-a4d1-c6f49f39d4fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_a24ee50b-8da6-4d8d-b6c4-a4804bce6661" xlink:to="loc_us-gaap_StatementClassOfStockAxis_acf9394f-663b-41b9-a4d1-c6f49f39d4fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_acf9394f-663b-41b9-a4d1-c6f49f39d4fa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_acf9394f-663b-41b9-a4d1-c6f49f39d4fa" xlink:to="loc_us-gaap_ClassOfStockDomain_acf9394f-663b-41b9-a4d1-c6f49f39d4fa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_430623e8-db0f-4599-b84b-5c9d2031879f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_acf9394f-663b-41b9-a4d1-c6f49f39d4fa" xlink:to="loc_us-gaap_ClassOfStockDomain_430623e8-db0f-4599-b84b-5c9d2031879f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_e325c5e3-170a-45ed-9057-e14375512a6d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_430623e8-db0f-4599-b84b-5c9d2031879f" xlink:to="loc_us-gaap_CommonClassAMember_e325c5e3-170a-45ed-9057-e14375512a6d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_be121b03-0aba-43a0-b04f-db5cc62c1040" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_430623e8-db0f-4599-b84b-5c9d2031879f" xlink:to="loc_us-gaap_CommonClassBMember_be121b03-0aba-43a0-b04f-db5cc62c1040" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_42df5dd9-f80c-41b3-8e52-503337a18bdc" xlink:to="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_c9648492-f78c-4835-ab26-e6d79daa1339" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:to="loc_us-gaap_AssetsCurrentAbstract_c9648492-f78c-4835-ab26-e6d79daa1339" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_570cd340-07dd-4dce-9e7b-c27b41c8f5f1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_c9648492-f78c-4835-ab26-e6d79daa1339" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_570cd340-07dd-4dce-9e7b-c27b41c8f5f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_3db3ae24-4f00-4165-a8d3-88e5a6ba6335" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_c9648492-f78c-4835-ab26-e6d79daa1339" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_3db3ae24-4f00-4165-a8d3-88e5a6ba6335" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SettlementAssetsCurrent_22d78c90-fb9d-48ba-9eca-9567731ba0b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SettlementAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_c9648492-f78c-4835-ab26-e6d79daa1339" xlink:to="loc_us-gaap_SettlementAssetsCurrent_22d78c90-fb9d-48ba-9eca-9567731ba0b0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_60bd6e86-cb82-4d1c-8a61-e7099e4c30b8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_c9648492-f78c-4835-ab26-e6d79daa1339" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_60bd6e86-cb82-4d1c-8a61-e7099e4c30b8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent_4697e9dc-216c-47d7-8691-78991e014f63" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_c9648492-f78c-4835-ab26-e6d79daa1339" xlink:to="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent_4697e9dc-216c-47d7-8691-78991e014f63" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_aae9655b-14c2-4901-8335-eb5017604142" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_c9648492-f78c-4835-ab26-e6d79daa1339" xlink:to="loc_us-gaap_AssetsCurrent_aae9655b-14c2-4901-8335-eb5017604142" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_09cb0e08-a95c-4dca-a071-3054a3ff60e7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_09cb0e08-a95c-4dca-a071-3054a3ff60e7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashNoncurrent_621facb9-f2f8-4fb8-99cb-18489c6d78f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:to="loc_us-gaap_RestrictedCashNoncurrent_621facb9-f2f8-4fb8-99cb-18489c6d78f7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedComputerSoftwareNet_16b389a5-646d-41d6-aa79-13f75169e2f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedComputerSoftwareNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:to="loc_us-gaap_CapitalizedComputerSoftwareNet_16b389a5-646d-41d6-aa79-13f75169e2f0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_281a6bbb-985a-498b-9d29-6142823086f6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:to="loc_us-gaap_Goodwill_281a6bbb-985a-498b-9d29-6142823086f6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_3b723c66-9340-4edf-bca8-cbb96604e97c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_3b723c66-9340-4edf-bca8-cbb96604e97c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxAssetsNet_aad81cb3-30ed-4af3-884c-4e1231f70e2b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:to="loc_us-gaap_DeferredIncomeTaxAssetsNet_aad81cb3-30ed-4af3-884c-4e1231f70e2b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_67eeaa68-6845-4ab9-8ac1-32a6cd924604" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_67eeaa68-6845-4ab9-8ac1-32a6cd924604" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_6b71c637-7312-4fad-9579-3eab57a8b2a2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_6b71c637-7312-4fad-9579-3eab57a8b2a2" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent_5098d1a9-e6f2-47f9-94c3-af4e0d0a20e7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent_5098d1a9-e6f2-47f9-94c3-af4e0d0a20e7" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_40b945f0-b299-4984-8848-e16a10250bda" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_56990752-f342-419d-aeae-951802430247" xlink:to="loc_us-gaap_Assets_40b945f0-b299-4984-8848-e16a10250bda" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_047fa271-06d7-49de-b46d-fb3f76e51306" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_42df5dd9-f80c-41b3-8e52-503337a18bdc" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_047fa271-06d7-49de-b46d-fb3f76e51306" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_f0d06ddc-c17b-4c09-af23-22f3463fb509" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_047fa271-06d7-49de-b46d-fb3f76e51306" xlink:to="loc_us-gaap_LiabilitiesAbstract_f0d06ddc-c17b-4c09-af23-22f3463fb509" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_1d2dba15-3937-429c-be11-27f1119bc718" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_f0d06ddc-c17b-4c09-af23-22f3463fb509" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_1d2dba15-3937-429c-be11-27f1119bc718" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_464f227b-53ce-48d1-834d-9eda847b1483" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_1d2dba15-3937-429c-be11-27f1119bc718" xlink:to="loc_us-gaap_AccountsPayableCurrent_464f227b-53ce-48d1-834d-9eda847b1483" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_107094fa-5de4-4d06-806d-2d7e243ba69c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_1d2dba15-3937-429c-be11-27f1119bc718" xlink:to="loc_us-gaap_LongTermDebtCurrent_107094fa-5de4-4d06-806d-2d7e243ba69c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent_a690b72c-90c3-4402-b734-25659f35a28b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_1d2dba15-3937-429c-be11-27f1119bc718" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent_a690b72c-90c3-4402-b734-25659f35a28b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SettlementLiabilitiesCurrent_ed74b86c-3fdb-43e3-8e8d-37a3a3092b67" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SettlementLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_1d2dba15-3937-429c-be11-27f1119bc718" xlink:to="loc_us-gaap_SettlementLiabilitiesCurrent_ed74b86c-3fdb-43e3-8e8d-37a3a3092b67" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_eb49b792-9b6e-4e83-8bec-b6e1b047c20f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_1d2dba15-3937-429c-be11-27f1119bc718" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_eb49b792-9b6e-4e83-8bec-b6e1b047c20f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_4a5635d1-0f43-4525-8798-cdcba9e3f97c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_1d2dba15-3937-429c-be11-27f1119bc718" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_4a5635d1-0f43-4525-8798-cdcba9e3f97c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent_64e9fe96-440d-4568-8b61-5bd7a2fabbe3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_1d2dba15-3937-429c-be11-27f1119bc718" xlink:to="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent_64e9fe96-440d-4568-8b61-5bd7a2fabbe3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_f97b9f7a-89d6-4fcc-ae2e-22cabea0f413" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_1d2dba15-3937-429c-be11-27f1119bc718" xlink:to="loc_us-gaap_LiabilitiesCurrent_f97b9f7a-89d6-4fcc-ae2e-22cabea0f413" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_TaxReceivableAgreementObligationNoncurrent_b9cf7e5f-9830-4645-b655-20305a470473" xlink:href="iiiv-20250630.xsd#iiiv_TaxReceivableAgreementObligationNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_f0d06ddc-c17b-4c09-af23-22f3463fb509" xlink:to="loc_iiiv_TaxReceivableAgreementObligationNoncurrent_b9cf7e5f-9830-4645-b655-20305a470473" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_2e2e76ab-2589-4145-9b67-fb055cde2cf1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_f0d06ddc-c17b-4c09-af23-22f3463fb509" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_2e2e76ab-2589-4145-9b67-fb055cde2cf1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_4a490d8d-5049-43fc-8b61-c5eaa314fd6b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_f0d06ddc-c17b-4c09-af23-22f3463fb509" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_4a490d8d-5049-43fc-8b61-c5eaa314fd6b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent_ca74e4be-20e6-4444-98a3-2b5d3aa9bd84" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_f0d06ddc-c17b-4c09-af23-22f3463fb509" xlink:to="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent_ca74e4be-20e6-4444-98a3-2b5d3aa9bd84" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_a92fb9d4-7b13-45f4-b48e-95935be1cdc8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_f0d06ddc-c17b-4c09-af23-22f3463fb509" xlink:to="loc_us-gaap_Liabilities_a92fb9d4-7b13-45f4-b48e-95935be1cdc8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_565af56b-9bbb-416a-87fd-e98b45591cf9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_047fa271-06d7-49de-b46d-fb3f76e51306" xlink:to="loc_us-gaap_CommitmentsAndContingencies_565af56b-9bbb-416a-87fd-e98b45591cf9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_328b2855-0bf4-470b-a5f8-15a0bbaeecd5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_047fa271-06d7-49de-b46d-fb3f76e51306" xlink:to="loc_us-gaap_StockholdersEquityAbstract_328b2855-0bf4-470b-a5f8-15a0bbaeecd5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockValue_a9040cbb-64e5-4f3c-87fc-653331d4804a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_328b2855-0bf4-470b-a5f8-15a0bbaeecd5" xlink:to="loc_us-gaap_PreferredStockValue_a9040cbb-64e5-4f3c-87fc-653331d4804a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_56907830-171b-4f3d-9db2-116cb9e01134" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_328b2855-0bf4-470b-a5f8-15a0bbaeecd5" xlink:to="loc_us-gaap_CommonStockValue_56907830-171b-4f3d-9db2-116cb9e01134" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapital_0a7cb6ce-a8e6-4835-a177-d6eb25cdd962" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_328b2855-0bf4-470b-a5f8-15a0bbaeecd5" xlink:to="loc_us-gaap_AdditionalPaidInCapital_0a7cb6ce-a8e6-4835-a177-d6eb25cdd962" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_a4f128e4-d7e5-4e1b-ba35-3c4784f23127" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_328b2855-0bf4-470b-a5f8-15a0bbaeecd5" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_a4f128e4-d7e5-4e1b-ba35-3c4784f23127" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_efd00d8e-a607-43cf-915b-d0322d783028" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_328b2855-0bf4-470b-a5f8-15a0bbaeecd5" xlink:to="loc_us-gaap_StockholdersEquity_efd00d8e-a607-43cf-915b-d0322d783028" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterest_f4a2daa8-06c4-4355-bd01-936dda4225fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_328b2855-0bf4-470b-a5f8-15a0bbaeecd5" xlink:to="loc_us-gaap_MinorityInterest_f4a2daa8-06c4-4355-bd01-936dda4225fe" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_ab477a39-f41a-4e87-be8c-948e5d06112e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_328b2855-0bf4-470b-a5f8-15a0bbaeecd5" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_ab477a39-f41a-4e87-be8c-948e5d06112e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_67ef14bf-17e2-4f7d-8932-f66c84bbd67d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_047fa271-06d7-49de-b46d-fb3f76e51306" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_67ef14bf-17e2-4f7d-8932-f66c84bbd67d" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSUNAUDITEDParenthetical" xlink:type="simple" xlink:href="iiiv-20250630.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETSUNAUDITEDParenthetical"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSUNAUDITEDParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_952e8738-a1dd-4365-a5b7-a43122052ac7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_6a5fb5bc-4961-401e-ae64-3deb5da0ca36" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_952e8738-a1dd-4365-a5b7-a43122052ac7" xlink:to="loc_us-gaap_StatementTable_6a5fb5bc-4961-401e-ae64-3deb5da0ca36" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_4b232848-a356-4f38-b67b-8972e7175fb4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_6a5fb5bc-4961-401e-ae64-3deb5da0ca36" xlink:to="loc_us-gaap_StatementClassOfStockAxis_4b232848-a356-4f38-b67b-8972e7175fb4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_4b232848-a356-4f38-b67b-8972e7175fb4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_4b232848-a356-4f38-b67b-8972e7175fb4" xlink:to="loc_us-gaap_ClassOfStockDomain_4b232848-a356-4f38-b67b-8972e7175fb4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_4dcdabb4-08f4-45a8-a063-5c5b2098bbc6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_4b232848-a356-4f38-b67b-8972e7175fb4" xlink:to="loc_us-gaap_ClassOfStockDomain_4dcdabb4-08f4-45a8-a063-5c5b2098bbc6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_bb0969c7-872b-4f0d-a8f5-c8a9dd30fde7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_4dcdabb4-08f4-45a8-a063-5c5b2098bbc6" xlink:to="loc_us-gaap_CommonClassAMember_bb0969c7-872b-4f0d-a8f5-c8a9dd30fde7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_926c6eee-19a4-44aa-8711-61673aeea993" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_4dcdabb4-08f4-45a8-a063-5c5b2098bbc6" xlink:to="loc_us-gaap_CommonClassBMember_926c6eee-19a4-44aa-8711-61673aeea993" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_e7f456a2-3ef5-458d-aa8a-9a9721346426" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_952e8738-a1dd-4365-a5b7-a43122052ac7" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_e7f456a2-3ef5-458d-aa8a-9a9721346426" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_ab09306a-8686-4401-a6fa-90576932adaf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_952e8738-a1dd-4365-a5b7-a43122052ac7" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_ab09306a-8686-4401-a6fa-90576932adaf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_71155d86-377f-4c9d-8b17-6962b12e45b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_952e8738-a1dd-4365-a5b7-a43122052ac7" xlink:to="loc_us-gaap_PreferredStockSharesIssued_71155d86-377f-4c9d-8b17-6962b12e45b3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_dfdb5c7b-4fbf-4221-9062-df716fa440c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_952e8738-a1dd-4365-a5b7-a43122052ac7" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_dfdb5c7b-4fbf-4221-9062-df716fa440c0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_eb080af0-d56b-4f93-8a5b-b339f10fb066" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_952e8738-a1dd-4365-a5b7-a43122052ac7" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_eb080af0-d56b-4f93-8a5b-b339f10fb066" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_42c94918-c3e5-49d0-ae40-7848434b8007" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_952e8738-a1dd-4365-a5b7-a43122052ac7" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_42c94918-c3e5-49d0-ae40-7848434b8007" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_7a540a20-6244-4fe8-8970-8cade249d5e1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_952e8738-a1dd-4365-a5b7-a43122052ac7" xlink:to="loc_us-gaap_CommonStockSharesIssued_7a540a20-6244-4fe8-8970-8cade249d5e1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_116c68dd-1a45-4f7a-beeb-d390c019e912" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_952e8738-a1dd-4365-a5b7-a43122052ac7" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_116c68dd-1a45-4f7a-beeb-d390c019e912" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONSUNAUDITED" xlink:type="simple" xlink:href="iiiv-20250630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONSUNAUDITED"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONSUNAUDITED" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_cbd2fd46-5041-41a4-83b4-d26422496308" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_StatementTable_cbd2fd46-5041-41a4-83b4-d26422496308" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_bb1c3b60-bf2a-4b7a-98d7-f5a91bc27ee4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementOperatingActivitiesSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_cbd2fd46-5041-41a4-83b4-d26422496308" xlink:to="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_bb1c3b60-bf2a-4b7a-98d7-f5a91bc27ee4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_bb1c3b60-bf2a-4b7a-98d7-f5a91bc27ee4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_bb1c3b60-bf2a-4b7a-98d7-f5a91bc27ee4" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_bb1c3b60-bf2a-4b7a-98d7-f5a91bc27ee4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_c0628567-17f6-48cb-ac8d-d0efb999bbb7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_bb1c3b60-bf2a-4b7a-98d7-f5a91bc27ee4" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_c0628567-17f6-48cb-ac8d-d0efb999bbb7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentContinuingOperationsMember_97cba916-b81e-44cb-9569-036476701ec7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentContinuingOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentOperatingActivitiesDomain_c0628567-17f6-48cb-ac8d-d0efb999bbb7" xlink:to="loc_us-gaap_SegmentContinuingOperationsMember_97cba916-b81e-44cb-9569-036476701ec7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDiscontinuedOperationsMember_4b260d53-2733-47dc-a473-47584190b60b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDiscontinuedOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentOperatingActivitiesDomain_c0628567-17f6-48cb-ac8d-d0efb999bbb7" xlink:to="loc_us-gaap_SegmentDiscontinuedOperationsMember_4b260d53-2733-47dc-a473-47584190b60b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_52e13a4c-db6f-46db-a24a-5859e6879b40" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_52e13a4c-db6f-46db-a24a-5859e6879b40" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpensesAbstract_743fb0cd-41ce-464c-9f02-474e83c9485c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_OperatingExpensesAbstract_743fb0cd-41ce-464c-9f02-474e83c9485c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCostOfOperatingRevenue_c4f66c42-f8b7-4166-8d6c-009138b8d40f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCostOfOperatingRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_743fb0cd-41ce-464c-9f02-474e83c9485c" xlink:to="loc_us-gaap_OtherCostOfOperatingRevenue_c4f66c42-f8b7-4166-8d6c-009138b8d40f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_e0144568-22d4-4af0-b3ec-56f7f4fd0a52" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_743fb0cd-41ce-464c-9f02-474e83c9485c" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_e0144568-22d4-4af0-b3ec-56f7f4fd0a52" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_d1abf48a-e8a0-4074-baf6-2e660151c8ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_743fb0cd-41ce-464c-9f02-474e83c9485c" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_d1abf48a-e8a0-4074-baf6-2e660151c8ec" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationIncludingDiscontinuedOperationsLiability_3d9848c0-a897-4a7c-ab44-6694017ab7e9" xlink:href="iiiv-20250630.xsd#iiiv_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationIncludingDiscontinuedOperationsLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_743fb0cd-41ce-464c-9f02-474e83c9485c" xlink:to="loc_iiiv_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationIncludingDiscontinuedOperationsLiability_3d9848c0-a897-4a7c-ab44-6694017ab7e9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostsAndExpenses_3b839011-623c-4cf2-91e2-54a52f1e58c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostsAndExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_743fb0cd-41ce-464c-9f02-474e83c9485c" xlink:to="loc_us-gaap_CostsAndExpenses_3b839011-623c-4cf2-91e2-54a52f1e58c8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_45a5a831-3416-452a-9e72-23ed72da4c54" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_OperatingIncomeLoss_45a5a831-3416-452a-9e72-23ed72da4c54" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_767a3e15-2730-45e5-9635-bcf7242f76ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNonoperatingIncomeExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_767a3e15-2730-45e5-9635-bcf7242f76ee" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_7f66eaf7-c3c8-4fc4-a3a5-7aba2f24f70f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestIncomeExpenseNonoperatingNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_767a3e15-2730-45e5-9635-bcf7242f76ee" xlink:to="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_7f66eaf7-c3c8-4fc4-a3a5-7aba2f24f70f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_dab5e007-57de-4110-992e-94b7e28bc8c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_767a3e15-2730-45e5-9635-bcf7242f76ee" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_dab5e007-57de-4110-992e-94b7e28bc8c1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_f2d0a4d9-d2d9-4246-b3de-797570b5a439" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_767a3e15-2730-45e5-9635-bcf7242f76ee" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_f2d0a4d9-d2d9-4246-b3de-797570b5a439" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_ca328f56-4e0a-4b46-a654-d93d769d1a71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_ca328f56-4e0a-4b46-a654-d93d769d1a71" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_924369cf-c014-4a26-84aa-3768034e3a1e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_924369cf-c014-4a26-84aa-3768034e3a1e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest_920f523d-6248-405d-b49f-2b3819c51457" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest_920f523d-6248-405d-b49f-2b3819c51457" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_2354f130-2ed3-4d4f-ac5a-856ebab6a540" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_2354f130-2ed3-4d4f-ac5a-856ebab6a540" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_c4b43180-7784-41e6-98a9-3917485ba198" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_ProfitLoss_c4b43180-7784-41e6-98a9-3917485ba198" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsAttributableToNoncontrollingEntity_eb923716-2b1f-4491-8f57-f3c72bd5fb8a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsAttributableToNoncontrollingEntity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsAttributableToNoncontrollingEntity_eb923716-2b1f-4491-8f57-f3c72bd5fb8a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToNoncontrollingInterest_ca7a9511-fa8f-4a40-8dc4-f8eed35a1bd5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToNoncontrollingInterest_ca7a9511-fa8f-4a40-8dc4-f8eed35a1bd5" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_8aa86474-ddbb-432e-ad89-b26b6218c696" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_8aa86474-ddbb-432e-ad89-b26b6218c696" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperations_d8feada2-3757-44fe-b30b-55daee78e753" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperations_d8feada2-3757-44fe-b30b-55daee78e753" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity_27132926-48d2-4ba9-9088-579e8a52d020" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity_27132926-48d2-4ba9-9088-579e8a52d020" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_9c51e007-94a9-49fd-8703-e0ad5f6e0458" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_NetIncomeLoss_9c51e007-94a9-49fd-8703-e0ad5f6e0458" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_NetIncomeLossPerShareAttributableToClassACommonStockholdersFromContinuingOperationsAbstract_3fb5e796-0225-4bd6-9e7d-e7252d1233c9" xlink:href="iiiv-20250630.xsd#iiiv_NetIncomeLossPerShareAttributableToClassACommonStockholdersFromContinuingOperationsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_iiiv_NetIncomeLossPerShareAttributableToClassACommonStockholdersFromContinuingOperationsAbstract_3fb5e796-0225-4bd6-9e7d-e7252d1233c9" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsPerBasicShare_eb47d768-9f45-4966-bb57-f115f8706fad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsPerBasicShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_NetIncomeLossPerShareAttributableToClassACommonStockholdersFromContinuingOperationsAbstract_3fb5e796-0225-4bd6-9e7d-e7252d1233c9" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsPerBasicShare_eb47d768-9f45-4966-bb57-f115f8706fad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare_7291faf9-76a8-49b8-b270-457704d5acb5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_NetIncomeLossPerShareAttributableToClassACommonStockholdersFromContinuingOperationsAbstract_3fb5e796-0225-4bd6-9e7d-e7252d1233c9" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare_7291faf9-76a8-49b8-b270-457704d5acb5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_NetIncomeLossPerShareAttributableToClassACommonStockholdersFromDiscontinuingOperationsAbstract_19e69550-0549-4f10-a75a-3b33ff5ae692" xlink:href="iiiv-20250630.xsd#iiiv_NetIncomeLossPerShareAttributableToClassACommonStockholdersFromDiscontinuingOperationsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_iiiv_NetIncomeLossPerShareAttributableToClassACommonStockholdersFromDiscontinuingOperationsAbstract_19e69550-0549-4f10-a75a-3b33ff5ae692" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare_5a47c678-c383-49b1-9807-e277b808c4ba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_NetIncomeLossPerShareAttributableToClassACommonStockholdersFromDiscontinuingOperationsAbstract_19e69550-0549-4f10-a75a-3b33ff5ae692" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare_5a47c678-c383-49b1-9807-e277b808c4ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare_3d3c42e5-cf83-4194-b4c0-3bccf6d5b9b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_NetIncomeLossPerShareAttributableToClassACommonStockholdersFromDiscontinuingOperationsAbstract_19e69550-0549-4f10-a75a-3b33ff5ae692" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare_3d3c42e5-cf83-4194-b4c0-3bccf6d5b9b2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_a9b674b4-e900-4bab-982e-da1421042e11" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a32c2e56-f7ed-47e2-a92e-c9349f07b32c" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_a9b674b4-e900-4bab-982e-da1421042e11" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_f827e79e-561b-45b4-a82c-5ffecc938de7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_a9b674b4-e900-4bab-982e-da1421042e11" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_f827e79e-561b-45b4-a82c-5ffecc938de7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_aaf3df43-93d6-4c6b-9349-fe99f2408d89" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_a9b674b4-e900-4bab-982e-da1421042e11" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_aaf3df43-93d6-4c6b-9349-fe99f2408d89" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/CONDENSEDCONSOLIDATEDSTATEMENTOFCHANGESINEQUITYUNAUDITED" xlink:type="simple" xlink:href="iiiv-20250630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTOFCHANGESINEQUITYUNAUDITED"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/CONDENSEDCONSOLIDATEDSTATEMENTOFCHANGESINEQUITYUNAUDITED" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_a0a3251e-98c2-4ec3-bc99-1d4430f80cd4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_e6a308df-f0ac-4730-b84d-1f5d9f8f9339" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_a0a3251e-98c2-4ec3-bc99-1d4430f80cd4" xlink:to="loc_us-gaap_StatementTable_e6a308df-f0ac-4730-b84d-1f5d9f8f9339" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_89c6e462-5766-4712-a146-774f4d8324ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_e6a308df-f0ac-4730-b84d-1f5d9f8f9339" xlink:to="loc_us-gaap_StatementClassOfStockAxis_89c6e462-5766-4712-a146-774f4d8324ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_89c6e462-5766-4712-a146-774f4d8324ec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_89c6e462-5766-4712-a146-774f4d8324ec" xlink:to="loc_us-gaap_ClassOfStockDomain_89c6e462-5766-4712-a146-774f4d8324ec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_73f139f1-add2-4621-ae38-6d79a633d0b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_89c6e462-5766-4712-a146-774f4d8324ec" xlink:to="loc_us-gaap_ClassOfStockDomain_73f139f1-add2-4621-ae38-6d79a633d0b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_4a854039-fdf7-42e7-956c-e9061e315ba0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_73f139f1-add2-4621-ae38-6d79a633d0b4" xlink:to="loc_us-gaap_CommonClassAMember_4a854039-fdf7-42e7-956c-e9061e315ba0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_834c1553-e047-4582-881e-17a93de0ea35" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_73f139f1-add2-4621-ae38-6d79a633d0b4" xlink:to="loc_us-gaap_CommonClassBMember_834c1553-e047-4582-881e-17a93de0ea35" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_a6d6141d-2c57-4dff-aece-949ab797ba23" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_e6a308df-f0ac-4730-b84d-1f5d9f8f9339" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_a6d6141d-2c57-4dff-aece-949ab797ba23" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_a6d6141d-2c57-4dff-aece-949ab797ba23_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_a6d6141d-2c57-4dff-aece-949ab797ba23" xlink:to="loc_us-gaap_EquityComponentDomain_a6d6141d-2c57-4dff-aece-949ab797ba23_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_a5ca25df-f1ec-419a-b7ce-ea6ead932cbe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_a6d6141d-2c57-4dff-aece-949ab797ba23" xlink:to="loc_us-gaap_EquityComponentDomain_a5ca25df-f1ec-419a-b7ce-ea6ead932cbe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_6363aeac-794f-4a17-92fc-7c642bc2c028" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_a5ca25df-f1ec-419a-b7ce-ea6ead932cbe" xlink:to="loc_us-gaap_CommonStockMember_6363aeac-794f-4a17-92fc-7c642bc2c028" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_263c4501-2095-4715-8c37-ef6b4e7dd51c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_a5ca25df-f1ec-419a-b7ce-ea6ead932cbe" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_263c4501-2095-4715-8c37-ef6b4e7dd51c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_d5c65165-e7fc-4aa7-a0a9-94d9fed31761" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_a5ca25df-f1ec-419a-b7ce-ea6ead932cbe" xlink:to="loc_us-gaap_RetainedEarningsMember_d5c65165-e7fc-4aa7-a0a9-94d9fed31761" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncontrollingInterestMember_bbf744d7-4bc9-4526-b662-c20e7a0e6ca6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncontrollingInterestMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_a5ca25df-f1ec-419a-b7ce-ea6ead932cbe" xlink:to="loc_us-gaap_NoncontrollingInterestMember_bbf744d7-4bc9-4526-b662-c20e7a0e6ca6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a0a3251e-98c2-4ec3-bc99-1d4430f80cd4" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_ad3855de-de4a-413d-923c-2788173124b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_ad3855de-de4a-413d-923c-2788173124b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_001ce649-a70d-4fbf-a396-465112b9e66a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_001ce649-a70d-4fbf-a396-465112b9e66a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_e73e5d7a-a2e5-4c4f-b34f-ae3750157324" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_e73e5d7a-a2e5-4c4f-b34f-ae3750157324" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_b01cda36-3481-4b54-a4e1-f355e61a94d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_us-gaap_ProfitLoss_b01cda36-3481-4b54-a4e1-f355e61a94d7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders_4a3b5861-a885-4f80-8bf0-f1bb87030e4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders_4a3b5861-a885-4f80-8bf0-f1bb87030e4a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AdjustmentsToAccruedDistributionsToNonControllingInterestHolders_4afac5e1-6143-480c-b395-6d97245ccbd6" xlink:href="iiiv-20250630.xsd#iiiv_AdjustmentsToAccruedDistributionsToNonControllingInterestHolders"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_iiiv_AdjustmentsToAccruedDistributionsToNonControllingInterestHolders_4afac5e1-6143-480c-b395-6d97245ccbd6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRedeemedOrCalledDuringPeriodShares_54e577ba-824f-4439-867d-bc44921ae075" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRedeemedOrCalledDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_us-gaap_StockRedeemedOrCalledDuringPeriodShares_54e577ba-824f-4439-867d-bc44921ae075" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRedeemedOrCalledDuringPeriodValue_a1e538a7-bb9f-4bf1-8641-d0137a4f7892" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRedeemedOrCalledDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_us-gaap_StockRedeemedOrCalledDuringPeriodValue_a1e538a7-bb9f-4bf1-8641-d0137a4f7892" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_StockIssuedDuringPeriodSharesRecapitalization_27a780b7-c259-467e-a70e-d63a517e8be2" xlink:href="iiiv-20250630.xsd#iiiv_StockIssuedDuringPeriodSharesRecapitalization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_iiiv_StockIssuedDuringPeriodSharesRecapitalization_27a780b7-c259-467e-a70e-d63a517e8be2" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_StockIssuedDuringPeriodValueRecapitalization_77e75819-154e-4ed5-9094-8ca48d79deb3" xlink:href="iiiv-20250630.xsd#iiiv_StockIssuedDuringPeriodValueRecapitalization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_iiiv_StockIssuedDuringPeriodValueRecapitalization_77e75819-154e-4ed5-9094-8ca48d79deb3" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AdjustmentsToAdditionalPaidInCapitalTaxRelatedAdjustments_ff4a7962-a4fe-47df-9845-98665882e7da" xlink:href="iiiv-20250630.xsd#iiiv_AdjustmentsToAdditionalPaidInCapitalTaxRelatedAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_iiiv_AdjustmentsToAdditionalPaidInCapitalTaxRelatedAdjustments_ff4a7962-a4fe-47df-9845-98665882e7da" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_2fe5d3fd-941e-4079-938d-e68b5ff2acee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_2fe5d3fd-941e-4079-938d-e68b5ff2acee" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition_b6236e7f-c6ec-4013-ba7c-2d9d1846f841" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition_b6236e7f-c6ec-4013-ba7c-2d9d1846f841" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AdjustmentsToAdditionalPaidInCapitalConvertibleDebtSaleOfHedges_4b4fe6d7-a2c2-48d6-97df-9e4418193a8a" xlink:href="iiiv-20250630.xsd#iiiv_AdjustmentsToAdditionalPaidInCapitalConvertibleDebtSaleOfHedges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_iiiv_AdjustmentsToAdditionalPaidInCapitalConvertibleDebtSaleOfHedges_4b4fe6d7-a2c2-48d6-97df-9e4418193a8a" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AdjustmentsToAdditionalPaidInCapitalWarrantsRepurchased_453b1f50-00a7-439a-8fc0-cec014757d5a" xlink:href="iiiv-20250630.xsd#iiiv_AdjustmentsToAdditionalPaidInCapitalWarrantsRepurchased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_iiiv_AdjustmentsToAdditionalPaidInCapitalWarrantsRepurchased_453b1f50-00a7-439a-8fc0-cec014757d5a" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodShares_72df34df-477d-4218-aaf7-70ba4393f8c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodShares_72df34df-477d-4218-aaf7-70ba4393f8c4" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodValue_23c13cb9-f582-4bf2-a476-9908d819b910" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodValue_23c13cb9-f582-4bf2-a476-9908d819b910" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AdjustmentsToAdditionalPaidInCapitalAllocationOfEquityToNoncontrollingInterest_a58d9b84-8713-4226-8bc6-e148539ac688" xlink:href="iiiv-20250630.xsd#iiiv_AdjustmentsToAdditionalPaidInCapitalAllocationOfEquityToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_2b473799-38ff-492f-8b0d-6f6817c37beb" xlink:to="loc_iiiv_AdjustmentsToAdditionalPaidInCapitalAllocationOfEquityToNoncontrollingInterest_a58d9b84-8713-4226-8bc6-e148539ac688" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_01d1cee0-e470-4bf9-848f-8ec5ce15fcef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_78e94a1c-fbef-49be-b873-ef7fecf530bc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUNAUDITED" xlink:type="simple" xlink:href="iiiv-20250630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUNAUDITED"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUNAUDITED" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_c35edfb1-2151-46a9-b5d6-c8f60714009c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_e368cfc5-71f4-4e6a-a2fd-231ef1e761c3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_c35edfb1-2151-46a9-b5d6-c8f60714009c" xlink:to="loc_us-gaap_StatementTable_e368cfc5-71f4-4e6a-a2fd-231ef1e761c3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_d0127835-cd8c-4062-9ef2-d2e9f56c7f5b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_e368cfc5-71f4-4e6a-a2fd-231ef1e761c3" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_d0127835-cd8c-4062-9ef2-d2e9f56c7f5b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_d0127835-cd8c-4062-9ef2-d2e9f56c7f5b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_d0127835-cd8c-4062-9ef2-d2e9f56c7f5b" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_d0127835-cd8c-4062-9ef2-d2e9f56c7f5b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_26e7fe6e-81fc-4525-9a4f-eea2069cbc02" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_d0127835-cd8c-4062-9ef2-d2e9f56c7f5b" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_26e7fe6e-81fc-4525-9a4f-eea2069cbc02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_MerchantServicesBusinessMember_9d06a253-a7f9-4c4e-a8cc-da60a83f5975" xlink:href="iiiv-20250630.xsd#iiiv_MerchantServicesBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_26e7fe6e-81fc-4525-9a4f-eea2069cbc02" xlink:to="loc_iiiv_MerchantServicesBusinessMember_9d06a253-a7f9-4c4e-a8cc-da60a83f5975" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_HealthcareRCMBusinessMember_b2d3d008-1d8e-49fd-9d03-0b45e4078192" xlink:href="iiiv-20250630.xsd#iiiv_HealthcareRCMBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_26e7fe6e-81fc-4525-9a4f-eea2069cbc02" xlink:to="loc_iiiv_HealthcareRCMBusinessMember_b2d3d008-1d8e-49fd-9d03-0b45e4078192" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_7f7075b7-93bb-41a0-9c6b-29d17872b1f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_c35edfb1-2151-46a9-b5d6-c8f60714009c" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_7f7075b7-93bb-41a0-9c6b-29d17872b1f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_49e18150-f1a5-4517-8d6f-7156bbe9e954" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_7f7075b7-93bb-41a0-9c6b-29d17872b1f7" xlink:to="loc_us-gaap_ProfitLoss_49e18150-f1a5-4517-8d6f-7156bbe9e954" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_7f7075b7-93bb-41a0-9c6b-29d17872b1f7" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DepreciationDepletionAndAmortizationIncludingDiscontinuedOperations_8a5cfebe-fefd-4e22-8fdd-7a3c5da66df7" xlink:href="iiiv-20250630.xsd#iiiv_DepreciationDepletionAndAmortizationIncludingDiscontinuedOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_iiiv_DepreciationDepletionAndAmortizationIncludingDiscontinuedOperations_8a5cfebe-fefd-4e22-8fdd-7a3c5da66df7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_15a85ca8-316c-4097-9502-d75ff1e1729b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_us-gaap_ShareBasedCompensation_15a85ca8-316c-4097-9502-d75ff1e1729b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_fe60710a-d959-4150-a9b3-291468d27b2e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_fe60710a-d959-4150-a9b3-291468d27b2e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnRepurchaseOfDebtInstrument_bcc96d84-bc9d-49d0-b7ac-cc4365614470" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnRepurchaseOfDebtInstrument"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_us-gaap_GainLossOnRepurchaseOfDebtInstrument_bcc96d84-bc9d-49d0-b7ac-cc4365614470" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfDerivatives_43a79ac2-3e0c-4f0c-96fa-44488ae3babb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSaleOfDerivatives"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_us-gaap_GainLossOnSaleOfDerivatives_43a79ac2-3e0c-4f0c-96fa-44488ae3babb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_GainLossOnRepurchasesOfWarrants_2b98c382-4085-4376-95b1-f0a1d8f38bc8" xlink:href="iiiv-20250630.xsd#iiiv_GainLossOnRepurchasesOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_iiiv_GainLossOnRepurchasesOfWarrants_2b98c382-4085-4376-95b1-f0a1d8f38bc8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_7a989a32-5770-41cd-b400-1776478fbe33" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_7a989a32-5770-41cd-b400-1776478fbe33" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfBusiness_d2eae628-25a7-4673-801c-2b9378b65410" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSaleOfBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_us-gaap_GainLossOnSaleOfBusiness_d2eae628-25a7-4673-801c-2b9378b65410" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_f61c1e2d-8067-4d34-8697-dbd8e29fb233" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_f61c1e2d-8067-4d34-8697-dbd8e29fb233" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_a0876e25-6734-4321-bb89-58663d66c7ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_a0876e25-6734-4321-bb89-58663d66c7ec" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncashIncomeExpense_e146e3dc-ccec-4983-95d2-a9889a19ed38" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNoncashIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_us-gaap_OtherNoncashIncomeExpense_e146e3dc-ccec-4983-95d2-a9889a19ed38" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingAssetsAbstract_7273b1b5-0601-4354-afba-fb06c95a7c90" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingAssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingAssetsAbstract_7273b1b5-0601-4354-afba-fb06c95a7c90" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_aaa06435-7dbc-423d-8941-6c5e95bce95c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingAssetsAbstract_7273b1b5-0601-4354-afba-fb06c95a7c90" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_aaa06435-7dbc-423d-8941-6c5e95bce95c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_e10387bd-b236-4e6a-95e9-05d2fa455d2d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingAssetsAbstract_7273b1b5-0601-4354-afba-fb06c95a7c90" xlink:to="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_e10387bd-b236-4e6a-95e9-05d2fa455d2d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssets_9e36f659-3e45-4352-ad40-8fa75b1391cb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOtherOperatingAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingAssetsAbstract_7273b1b5-0601-4354-afba-fb06c95a7c90" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssets_9e36f659-3e45-4352-ad40-8fa75b1391cb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingLiabilitiesAbstract_098ec166-3af4-4175-a725-963cc62fc790" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingLiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_04626ea7-74d6-4a64-b21e-5f533a441f2c" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingLiabilitiesAbstract_098ec166-3af4-4175-a725-963cc62fc790" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayableTrade_39f7936e-7219-4ab6-8d80-d6bb4f6c753b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInAccountsPayableTrade"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingLiabilitiesAbstract_098ec166-3af4-4175-a725-963cc62fc790" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayableTrade_39f7936e-7219-4ab6-8d80-d6bb4f6c753b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_a7614887-5973-44de-a229-106a46f67be1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingLiabilitiesAbstract_098ec166-3af4-4175-a725-963cc62fc790" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_a7614887-5973-44de-a229-106a46f67be1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_IncreaseDecreaseInAcquisitionEscrowObligations_65c8310f-a495-4cce-800c-f312eb1bec76" xlink:href="iiiv-20250630.xsd#iiiv_IncreaseDecreaseInAcquisitionEscrowObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingLiabilitiesAbstract_098ec166-3af4-4175-a725-963cc62fc790" xlink:to="loc_iiiv_IncreaseDecreaseInAcquisitionEscrowObligations_65c8310f-a495-4cce-800c-f312eb1bec76" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredRevenue_7dc5228f-3d7f-4cf7-ae5b-a532532ddfe1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInDeferredRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingLiabilitiesAbstract_098ec166-3af4-4175-a725-963cc62fc790" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredRevenue_7dc5228f-3d7f-4cf7-ae5b-a532532ddfe1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_IncreaseDecreaseInOperatingLeaseLiabilities_3dceb7d1-dda4-451c-8d18-b62dcb490d3e" xlink:href="iiiv-20250630.xsd#iiiv_IncreaseDecreaseInOperatingLeaseLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingLiabilitiesAbstract_098ec166-3af4-4175-a725-963cc62fc790" xlink:to="loc_iiiv_IncreaseDecreaseInOperatingLeaseLiabilities_3dceb7d1-dda4-451c-8d18-b62dcb490d3e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherNoncurrentLiabilities_a8746ed3-6b0e-422b-9d65-0a674bd9e772" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOtherNoncurrentLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingLiabilitiesAbstract_098ec166-3af4-4175-a725-963cc62fc790" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherNoncurrentLiabilities_a8746ed3-6b0e-422b-9d65-0a674bd9e772" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentForContingentConsiderationLiabilityOperatingActivities_f954d125-2a49-4907-9c5d-a0ede47ef16e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentForContingentConsiderationLiabilityOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingLiabilitiesAbstract_098ec166-3af4-4175-a725-963cc62fc790" xlink:to="loc_us-gaap_PaymentForContingentConsiderationLiabilityOperatingActivities_f954d125-2a49-4907-9c5d-a0ede47ef16e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_53477bab-84eb-4b18-b20d-e515cd5e84d8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_7f7075b7-93bb-41a0-9c6b-29d17872b1f7" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_53477bab-84eb-4b18-b20d-e515cd5e84d8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_552f60d4-0ea8-465d-9ac2-bfbca9316e05" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_c35edfb1-2151-46a9-b5d6-c8f60714009c" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_552f60d4-0ea8-465d-9ac2-bfbca9316e05" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_d916f80f-cd6d-4ce9-80ac-a69051adb3e2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_552f60d4-0ea8-465d-9ac2-bfbca9316e05" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_d916f80f-cd6d-4ce9-80ac-a69051adb3e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment_335d5e2e-5c78-465b-9474-63ed381b4a55" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_552f60d4-0ea8-465d-9ac2-bfbca9316e05" xlink:to="loc_us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment_335d5e2e-5c78-465b-9474-63ed381b4a55" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForSoftware_b8aae71c-f16f-4fca-b1d7-d4c4bbb9d69d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForSoftware"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_552f60d4-0ea8-465d-9ac2-bfbca9316e05" xlink:to="loc_us-gaap_PaymentsForSoftware_b8aae71c-f16f-4fca-b1d7-d4c4bbb9d69d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireIntangibleAssets_8e238e66-c14b-4c1b-b9e3-73f8dd899c91" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_552f60d4-0ea8-465d-9ac2-bfbca9316e05" xlink:to="loc_us-gaap_PaymentsToAcquireIntangibleAssets_8e238e66-c14b-4c1b-b9e3-73f8dd899c91" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_a07de915-a1be-4689-ab07-b45920bfb8c3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_552f60d4-0ea8-465d-9ac2-bfbca9316e05" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_a07de915-a1be-4689-ab07-b45920bfb8c3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested_70f2567c-12cb-48d6-92c7-58367e3bc690" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_552f60d4-0ea8-465d-9ac2-bfbca9316e05" xlink:to="loc_us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested_70f2567c-12cb-48d6-92c7-58367e3bc690" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_PaymentsForOtherInvestingActivities_13ad61fa-613d-45c8-a828-04348ac3cdc8" xlink:href="iiiv-20250630.xsd#iiiv_PaymentsForOtherInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_552f60d4-0ea8-465d-9ac2-bfbca9316e05" xlink:to="loc_iiiv_PaymentsForOtherInvestingActivities_13ad61fa-613d-45c8-a828-04348ac3cdc8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_ProceedsFromOtherInvestingActivities_a341fa0f-652b-4821-aeb4-d9053271232f" xlink:href="iiiv-20250630.xsd#iiiv_ProceedsFromOtherInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_552f60d4-0ea8-465d-9ac2-bfbca9316e05" xlink:to="loc_iiiv_ProceedsFromOtherInvestingActivities_a341fa0f-652b-4821-aeb4-d9053271232f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_49bcb6de-688f-440a-8d7d-c5000971d757" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_552f60d4-0ea8-465d-9ac2-bfbca9316e05" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_49bcb6de-688f-440a-8d7d-c5000971d757" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_c35edfb1-2151-46a9-b5d6-c8f60714009c" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromLinesOfCredit_9b19283a-97e3-4dff-86a5-f2b1f75995b1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromLinesOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_us-gaap_ProceedsFromLinesOfCredit_9b19283a-97e3-4dff-86a5-f2b1f75995b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLinesOfCredit_0e5ce48d-2f9a-4baf-9401-c269b2ca6b51" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfLinesOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_us-gaap_RepaymentsOfLinesOfCredit_0e5ce48d-2f9a-4baf-9401-c269b2ca6b51" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_PaymentsToExtinguishExchangeableNotes_6e5c6174-2840-47e3-b578-484e5d6d5fab" xlink:href="iiiv-20250630.xsd#iiiv_PaymentsToExtinguishExchangeableNotes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_iiiv_PaymentsToExtinguishExchangeableNotes_6e5c6174-2840-47e3-b578-484e5d6d5fab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfConvertibleDebt_70c7169a-22e8-4abf-ae12-c710f161b9ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfConvertibleDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_us-gaap_RepaymentsOfConvertibleDebt_70c7169a-22e8-4abf-ae12-c710f161b9ec" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromConvertibleDebt_e9847907-327f-4d41-a7ee-fc8de80c2860" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromConvertibleDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_us-gaap_ProceedsFromConvertibleDebt_e9847907-327f-4d41-a7ee-fc8de80c2860" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForRepurchaseOfWarrants_aad34bb2-c62a-4a48-b5b1-e1e4e84c68e4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForRepurchaseOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_us-gaap_PaymentsForRepurchaseOfWarrants_aad34bb2-c62a-4a48-b5b1-e1e4e84c68e4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDebtIssuanceCosts_692cfcf0-66fc-40d4-8d8f-e214da2db036" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDebtIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_us-gaap_PaymentsOfDebtIssuanceCosts_692cfcf0-66fc-40d4-8d8f-e214da2db036" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForRepurchaseOfCommonStock_f5084dab-98d9-4de4-9744-144cea00fb37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForRepurchaseOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_us-gaap_PaymentsForRepurchaseOfCommonStock_f5084dab-98d9-4de4-9744-144cea00fb37" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_NetPaymentsForProceedsFromSettlementObligations_90422e54-50d2-43dd-9a16-52eabc70ee8c" xlink:href="iiiv-20250630.xsd#iiiv_NetPaymentsForProceedsFromSettlementObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_iiiv_NetPaymentsForProceedsFromSettlementObligations_90422e54-50d2-43dd-9a16-52eabc70ee8c" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentForContingentConsiderationLiabilityFinancingActivities_bae6b351-dd9f-4e9d-b430-8c02850e454b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentForContingentConsiderationLiabilityFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_us-gaap_PaymentForContingentConsiderationLiabilityFinancingActivities_bae6b351-dd9f-4e9d-b430-8c02850e454b" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_PaymentsForRequiredDistributionsToMembersForTaxObligations_8f809c89-9a15-40a4-b04d-03e19d0c5a92" xlink:href="iiiv-20250630.xsd#iiiv_PaymentsForRequiredDistributionsToMembersForTaxObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_iiiv_PaymentsForRequiredDistributionsToMembersForTaxObligations_8f809c89-9a15-40a4-b04d-03e19d0c5a92" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDistributionsToAffiliates_444e8f35-b6bc-4445-9a3f-d342dae83e86" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDistributionsToAffiliates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_us-gaap_PaymentsOfDistributionsToAffiliates_444e8f35-b6bc-4445-9a3f-d342dae83e86" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromStockOptionsExercised_0603a556-116d-4471-a19b-b759763ffced" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_us-gaap_ProceedsFromStockOptionsExercised_0603a556-116d-4471-a19b-b759763ffced" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_d929f305-8a33-443c-b4d6-7424c783904b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_d929f305-8a33-443c-b4d6-7424c783904b" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_2d67c5fd-2e5f-4ff0-9d90-9f5437fa3424" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_8511b29c-661a-474f-9205-f1f888afeecb" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_2d67c5fd-2e5f-4ff0-9d90-9f5437fa3424" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_405d601b-e5b7-452c-aae5-ef00f24490b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_c35edfb1-2151-46a9-b5d6-c8f60714009c" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_405d601b-e5b7-452c-aae5-ef00f24490b2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_c3963852-8d67-4213-87dd-542a0a8f3723" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_c35edfb1-2151-46a9-b5d6-c8f60714009c" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_c3963852-8d67-4213-87dd-542a0a8f3723" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_8370940a-f01f-4e2f-9ea2-65f984529ca1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplementalCashFlowInformationAbstract_da9115a3-4cd8-4dd8-811c-16953fce1348" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SupplementalCashFlowInformationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_c35edfb1-2151-46a9-b5d6-c8f60714009c" xlink:to="loc_us-gaap_SupplementalCashFlowInformationAbstract_da9115a3-4cd8-4dd8-811c-16953fce1348" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPaidNet_2b987abb-9afe-4716-825f-e5f4076bf6ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_da9115a3-4cd8-4dd8-811c-16953fce1348" xlink:to="loc_us-gaap_InterestPaidNet_2b987abb-9afe-4716-825f-e5f4076bf6ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaid_4a1f3dbb-0a45-4806-bcad-6726f52b83d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxesPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_da9115a3-4cd8-4dd8-811c-16953fce1348" xlink:to="loc_us-gaap_IncomeTaxesPaid_4a1f3dbb-0a45-4806-bcad-6726f52b83d6" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/ACQUISITIONSTables" xlink:type="simple" xlink:href="iiiv-20250630.xsd#ACQUISITIONSTables"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/ACQUISITIONSTables" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_b3ed8de1-ff01-4c88-80ef-66004f0bbad9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3db14abc-5499-42e6-97bf-0827542127c9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b3ed8de1-ff01-4c88-80ef-66004f0bbad9" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3db14abc-5499-42e6-97bf-0827542127c9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_d7c573aa-ff2a-4419-b418-de2a3660b9b6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3db14abc-5499-42e6-97bf-0827542127c9" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_d7c573aa-ff2a-4419-b418-de2a3660b9b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_d7c573aa-ff2a-4419-b418-de2a3660b9b6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_d7c573aa-ff2a-4419-b418-de2a3660b9b6" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_d7c573aa-ff2a-4419-b418-de2a3660b9b6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_545edb82-f121-41e3-95b4-a3f3610b5543" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_d7c573aa-ff2a-4419-b418-de2a3660b9b6" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_545edb82-f121-41e3-95b4-a3f3610b5543" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_UtilityBillingSoftwareCompanyMember_2029514c-5381-4047-ac37-6e50743f97d3" xlink:href="iiiv-20250630.xsd#iiiv_UtilityBillingSoftwareCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_545edb82-f121-41e3-95b4-a3f3610b5543" xlink:to="loc_iiiv_UtilityBillingSoftwareCompanyMember_2029514c-5381-4047-ac37-6e50743f97d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_EdulokaLtd.Member_e689fed9-7e64-482a-8d07-825c51b623c5" xlink:href="iiiv-20250630.xsd#iiiv_EdulokaLtd.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_545edb82-f121-41e3-95b4-a3f3610b5543" xlink:to="loc_iiiv_EdulokaLtd.Member_e689fed9-7e64-482a-8d07-825c51b623c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRecognizedIdentifiedAssetsAcquiredAndLiabilitiesAssumedTableTextBlock_f7030cc7-d056-42a9-8ab2-d6cf9d27d5ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRecognizedIdentifiedAssetsAcquiredAndLiabilitiesAssumedTableTextBlock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b3ed8de1-ff01-4c88-80ef-66004f0bbad9" xlink:to="loc_us-gaap_ScheduleOfRecognizedIdentifiedAssetsAcquiredAndLiabilitiesAssumedTableTextBlock_f7030cc7-d056-42a9-8ab2-d6cf9d27d5ee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionProFormaInformationTextBlock_14d8f46e-9e3b-44fb-8aa0-38b45f8bc627" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionProFormaInformationTextBlock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b3ed8de1-ff01-4c88-80ef-66004f0bbad9" xlink:to="loc_us-gaap_BusinessAcquisitionProFormaInformationTextBlock_14d8f46e-9e3b-44fb-8aa0-38b45f8bc627" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/DISCONTINUEDOPERATIONSNarrativeDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#DISCONTINUEDOPERATIONSNarrativeDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/DISCONTINUEDOPERATIONSNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fde3849b-4030-4c79-afbc-4edd45431c97" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_6b54f022-0df7-4747-99f0-93930a668f80" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fde3849b-4030-4c79-afbc-4edd45431c97" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_6b54f022-0df7-4747-99f0-93930a668f80" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_83491001-9cba-4480-9d49-2394b3559dc1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_6b54f022-0df7-4747-99f0-93930a668f80" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_83491001-9cba-4480-9d49-2394b3559dc1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_83491001-9cba-4480-9d49-2394b3559dc1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_83491001-9cba-4480-9d49-2394b3559dc1" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_83491001-9cba-4480-9d49-2394b3559dc1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_2eecd98d-bacb-4c31-84fb-6fe26b53a2d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_83491001-9cba-4480-9d49-2394b3559dc1" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_2eecd98d-bacb-4c31-84fb-6fe26b53a2d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_HealthcareRCMBusinessMember_161b3a64-b0c8-42f8-8f74-ee73d0f1dddd" xlink:href="iiiv-20250630.xsd#iiiv_HealthcareRCMBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_2eecd98d-bacb-4c31-84fb-6fe26b53a2d7" xlink:to="loc_iiiv_HealthcareRCMBusinessMember_161b3a64-b0c8-42f8-8f74-ee73d0f1dddd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_MerchantServicesBusinessMember_6e280ca5-e2e3-4094-8455-716846a0e327" xlink:href="iiiv-20250630.xsd#iiiv_MerchantServicesBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_2eecd98d-bacb-4c31-84fb-6fe26b53a2d7" xlink:to="loc_iiiv_MerchantServicesBusinessMember_6e280ca5-e2e3-4094-8455-716846a0e327" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_1e5705b0-a108-43c4-9bff-c01682a8709d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_6b54f022-0df7-4747-99f0-93930a668f80" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_1e5705b0-a108-43c4-9bff-c01682a8709d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_1e5705b0-a108-43c4-9bff-c01682a8709d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_1e5705b0-a108-43c4-9bff-c01682a8709d" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_1e5705b0-a108-43c4-9bff-c01682a8709d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_7b49a263-cc2d-46ee-8506-e6f466488f21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_1e5705b0-a108-43c4-9bff-c01682a8709d" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_7b49a263-cc2d-46ee-8506-e6f466488f21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationsDisposedOfBySaleMember_c8dfce9a-bcaf-4235-b81f-ffc752baf917" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationsDisposedOfBySaleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_7b49a263-cc2d-46ee-8506-e6f466488f21" xlink:to="loc_us-gaap_DiscontinuedOperationsDisposedOfBySaleMember_c8dfce9a-bcaf-4235-b81f-ffc752baf917" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember_bf5f9e28-cce1-4c5d-a466-d3d56cbe2019" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_7b49a263-cc2d-46ee-8506-e6f466488f21" xlink:to="loc_us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember_bf5f9e28-cce1-4c5d-a466-d3d56cbe2019" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_33e52d46-4cd5-4b30-9314-0e79a966ea25" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_6b54f022-0df7-4747-99f0-93930a668f80" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_33e52d46-4cd5-4b30-9314-0e79a966ea25" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_33e52d46-4cd5-4b30-9314-0e79a966ea25_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_33e52d46-4cd5-4b30-9314-0e79a966ea25" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_33e52d46-4cd5-4b30-9314-0e79a966ea25_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_47010546-177a-40fa-a1ea-9be6e4bf4abc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_33e52d46-4cd5-4b30-9314-0e79a966ea25" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_47010546-177a-40fa-a1ea-9be6e4bf4abc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_InfinxTSAAndInfinxELAMember_ee26e20d-9b46-403d-8aed-482b6bdcecb7" xlink:href="iiiv-20250630.xsd#iiiv_InfinxTSAAndInfinxELAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_47010546-177a-40fa-a1ea-9be6e4bf4abc" xlink:to="loc_iiiv_InfinxTSAAndInfinxELAMember_ee26e20d-9b46-403d-8aed-482b6bdcecb7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_PayrocTSAAndPayrocPSAMember_d4597271-91d3-45ae-bbf3-8faaf22bbc1a" xlink:href="iiiv-20250630.xsd#iiiv_PayrocTSAAndPayrocPSAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_47010546-177a-40fa-a1ea-9be6e4bf4abc" xlink:to="loc_iiiv_PayrocTSAAndPayrocPSAMember_d4597271-91d3-45ae-bbf3-8faaf22bbc1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_27a8f1a8-12f1-4404-9d9d-524a3d47507f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_6b54f022-0df7-4747-99f0-93930a668f80" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_27a8f1a8-12f1-4404-9d9d-524a3d47507f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_27a8f1a8-12f1-4404-9d9d-524a3d47507f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_27a8f1a8-12f1-4404-9d9d-524a3d47507f" xlink:to="loc_us-gaap_RelatedPartyDomain_27a8f1a8-12f1-4404-9d9d-524a3d47507f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_193a55ef-8499-4f3f-98db-f8c578010355" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_27a8f1a8-12f1-4404-9d9d-524a3d47507f" xlink:to="loc_us-gaap_RelatedPartyDomain_193a55ef-8499-4f3f-98db-f8c578010355" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_4c8997fe-4361-4809-9de5-baffcaac6daf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_193a55ef-8499-4f3f-98db-f8c578010355" xlink:to="loc_us-gaap_RelatedPartyMember_4c8997fe-4361-4809-9de5-baffcaac6daf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromDivestitureOfBusinesses_67dd6cc2-68ec-4a32-9cbd-37f1980ddd52" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromDivestitureOfBusinesses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fde3849b-4030-4c79-afbc-4edd45431c97" xlink:to="loc_us-gaap_ProceedsFromDivestitureOfBusinesses_67dd6cc2-68ec-4a32-9cbd-37f1980ddd52" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_e923d7db-0de8-427a-9bfb-8e77b8d10e52" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fde3849b-4030-4c79-afbc-4edd45431c97" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_e923d7db-0de8-427a-9bfb-8e77b8d10e52" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationTransactionCostIncurred_7d4c5769-ecc9-48e7-9f20-46e1220443a9" xlink:href="iiiv-20250630.xsd#iiiv_DisposalGroupIncludingDiscontinuedOperationTransactionCostIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fde3849b-4030-4c79-afbc-4edd45431c97" xlink:to="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationTransactionCostIncurred_7d4c5769-ecc9-48e7-9f20-46e1220443a9" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/DISCONTINUEDOPERATIONSAssetsandLiabilitiesofDiscontinuedOperationsDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#DISCONTINUEDOPERATIONSAssetsandLiabilitiesofDiscontinuedOperationsDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/DISCONTINUEDOPERATIONSAssetsandLiabilitiesofDiscontinuedOperationsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_e06fe99b-cd77-4cd5-94b4-729c77f660ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_e848793d-8609-448a-b3c5-d6b953c80e84" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_e06fe99b-cd77-4cd5-94b4-729c77f660ee" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_e848793d-8609-448a-b3c5-d6b953c80e84" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_3adf5877-7d6a-4920-83aa-933dd515bec8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_e848793d-8609-448a-b3c5-d6b953c80e84" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_3adf5877-7d6a-4920-83aa-933dd515bec8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_3adf5877-7d6a-4920-83aa-933dd515bec8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_3adf5877-7d6a-4920-83aa-933dd515bec8" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_3adf5877-7d6a-4920-83aa-933dd515bec8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e278e769-de01-4658-9b3b-3fa9e179ef06" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_3adf5877-7d6a-4920-83aa-933dd515bec8" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e278e769-de01-4658-9b3b-3fa9e179ef06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_HealthcareRCMBusinessMember_23e9d2ab-a112-4f38-a107-d37866d1329c" xlink:href="iiiv-20250630.xsd#iiiv_HealthcareRCMBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e278e769-de01-4658-9b3b-3fa9e179ef06" xlink:to="loc_iiiv_HealthcareRCMBusinessMember_23e9d2ab-a112-4f38-a107-d37866d1329c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_20ded358-4db3-42b6-b668-d3ca8ac7097c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_e848793d-8609-448a-b3c5-d6b953c80e84" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_20ded358-4db3-42b6-b668-d3ca8ac7097c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_20ded358-4db3-42b6-b668-d3ca8ac7097c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_20ded358-4db3-42b6-b668-d3ca8ac7097c" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_20ded358-4db3-42b6-b668-d3ca8ac7097c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_952b1d04-a1cd-48f6-91f6-8179a13e5fed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_20ded358-4db3-42b6-b668-d3ca8ac7097c" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_952b1d04-a1cd-48f6-91f6-8179a13e5fed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember_1de4ece4-9c6c-4b4a-b90d-7f06edef9726" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_952b1d04-a1cd-48f6-91f6-8179a13e5fed" xlink:to="loc_us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember_1de4ece4-9c6c-4b4a-b90d-7f06edef9726" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationAbstract_51e42c3e-78ad-4063-bbd0-af4ae5f0af94" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_e06fe99b-cd77-4cd5-94b4-729c77f660ee" xlink:to="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationAbstract_51e42c3e-78ad-4063-bbd0-af4ae5f0af94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_885e2f99-e17c-4968-84c8-5f18e54af747" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationAbstract_51e42c3e-78ad-4063-bbd0-af4ae5f0af94" xlink:to="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_885e2f99-e17c-4968-84c8-5f18e54af747" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationCashAndCashEquivalents_f0a3edea-b9de-4b78-b2b2-a847988a4f1a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationCashAndCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_885e2f99-e17c-4968-84c8-5f18e54af747" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationCashAndCashEquivalents_f0a3edea-b9de-4b78-b2b2-a847988a4f1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationAccountsNotesAndLoansReceivableNet_b3353332-7f2c-40d1-b580-924f856a37b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationAccountsNotesAndLoansReceivableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_885e2f99-e17c-4968-84c8-5f18e54af747" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationAccountsNotesAndLoansReceivableNet_b3353332-7f2c-40d1-b580-924f856a37b9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationPrepaidAndOtherAssetsCurrent_f3217544-1a28-43dd-9fc8-040e5f7da381" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationPrepaidAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_885e2f99-e17c-4968-84c8-5f18e54af747" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationPrepaidAndOtherAssetsCurrent_f3217544-1a28-43dd-9fc8-040e5f7da381" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent_12d8dfda-4a89-4b51-9b10-67908b17ebe7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_885e2f99-e17c-4968-84c8-5f18e54af747" xlink:to="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent_12d8dfda-4a89-4b51-9b10-67908b17ebe7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment_17ccef63-f2a3-4b63-b2f5-dca8bf488503" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationAbstract_51e42c3e-78ad-4063-bbd0-af4ae5f0af94" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment_17ccef63-f2a3-4b63-b2f5-dca8bf488503" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationCapitalizedSoftwareNetNoncurrent_cc893c24-e1b9-4994-8be1-e3a2e6cc02e4" xlink:href="iiiv-20250630.xsd#iiiv_DisposalGroupIncludingDiscontinuedOperationCapitalizedSoftwareNetNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationAbstract_51e42c3e-78ad-4063-bbd0-af4ae5f0af94" xlink:to="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationCapitalizedSoftwareNetNoncurrent_cc893c24-e1b9-4994-8be1-e3a2e6cc02e4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationGoodwillNoncurrent_a29af780-64ff-4958-b185-850344602ee3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationGoodwillNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationAbstract_51e42c3e-78ad-4063-bbd0-af4ae5f0af94" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationGoodwillNoncurrent_a29af780-64ff-4958-b185-850344602ee3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationIntangibleAssetsNoncurrent_0c714284-ab29-47e5-8f99-ed8757565900" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationIntangibleAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationAbstract_51e42c3e-78ad-4063-bbd0-af4ae5f0af94" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationIntangibleAssetsNoncurrent_0c714284-ab29-47e5-8f99-ed8757565900" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingLeaseRightOfUseAssets_e76a5098-6e56-4250-ad1f-0763cfbd0835" xlink:href="iiiv-20250630.xsd#iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingLeaseRightOfUseAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationAbstract_51e42c3e-78ad-4063-bbd0-af4ae5f0af94" xlink:to="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingLeaseRightOfUseAssets_e76a5098-6e56-4250-ad1f-0763cfbd0835" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherNoncurrentAssets_5b7892bd-65f1-428c-b6ce-63a58abcea95" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherNoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationAbstract_51e42c3e-78ad-4063-bbd0-af4ae5f0af94" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherNoncurrentAssets_5b7892bd-65f1-428c-b6ce-63a58abcea95" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperation_bae474a7-6f85-487d-a695-4d2d30b4c9a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationAbstract_51e42c3e-78ad-4063-bbd0-af4ae5f0af94" xlink:to="loc_us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperation_bae474a7-6f85-487d-a695-4d2d30b4c9a0" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationAbstract_134138d0-55c6-4b4d-a035-d8462d75d7d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_e06fe99b-cd77-4cd5-94b4-729c77f660ee" xlink:to="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationAbstract_134138d0-55c6-4b4d-a035-d8462d75d7d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_198c2702-1ea2-48cd-b9b4-a2e2ec719027" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationAbstract_134138d0-55c6-4b4d-a035-d8462d75d7d0" xlink:to="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_198c2702-1ea2-48cd-b9b4-a2e2ec719027" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationAccountsPayableCurrent_f5affae1-106c-4bd0-88d8-6793c91cf34e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationAccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_198c2702-1ea2-48cd-b9b4-a2e2ec719027" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationAccountsPayableCurrent_f5affae1-106c-4bd0-88d8-6793c91cf34e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationAccruedLiabilitiesCurrent_498655cb-3899-4638-bde5-241dfb5d936f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationAccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_198c2702-1ea2-48cd-b9b4-a2e2ec719027" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationAccruedLiabilitiesCurrent_498655cb-3899-4638-bde5-241dfb5d936f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationDeferredRevenueCurrent_6c0a43ea-4912-41e5-8a23-ccb46a660884" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationDeferredRevenueCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_198c2702-1ea2-48cd-b9b4-a2e2ec719027" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationDeferredRevenueCurrent_6c0a43ea-4912-41e5-8a23-ccb46a660884" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingLeaseLiabilitiesCurrent_95949f86-614e-46d3-8eb3-55601a44bf6a" xlink:href="iiiv-20250630.xsd#iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingLeaseLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_198c2702-1ea2-48cd-b9b4-a2e2ec719027" xlink:to="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingLeaseLiabilitiesCurrent_95949f86-614e-46d3-8eb3-55601a44bf6a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent_99c2dc25-6d96-445a-baf7-81e6a4c45684" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrentAbstract_198c2702-1ea2-48cd-b9b4-a2e2ec719027" xlink:to="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent_99c2dc25-6d96-445a-baf7-81e6a4c45684" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingLeaseLiabilitiesNoncurrent_ba500876-a717-46b5-8ec0-e47aefffd478" xlink:href="iiiv-20250630.xsd#iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingLeaseLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationAbstract_134138d0-55c6-4b4d-a035-d8462d75d7d0" xlink:to="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingLeaseLiabilitiesNoncurrent_ba500876-a717-46b5-8ec0-e47aefffd478" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperation_4ca19dec-6734-4b7c-82ba-e5e4e1da342a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationAbstract_134138d0-55c6-4b4d-a035-d8462d75d7d0" xlink:to="loc_us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperation_4ca19dec-6734-4b7c-82ba-e5e4e1da342a" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/DISCONTINUEDOPERATIONSIncomeLossFromDiscontinuedOperationsDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#DISCONTINUEDOPERATIONSIncomeLossFromDiscontinuedOperationsDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/DISCONTINUEDOPERATIONSIncomeLossFromDiscontinuedOperationsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_2ad76897-1c87-43d6-81be-98c6986fc527" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_2ad76897-1c87-43d6-81be-98c6986fc527" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_99fda09c-e198-4fda-bba0-b93e100cab98" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_2ad76897-1c87-43d6-81be-98c6986fc527" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_99fda09c-e198-4fda-bba0-b93e100cab98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_99fda09c-e198-4fda-bba0-b93e100cab98_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_99fda09c-e198-4fda-bba0-b93e100cab98" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_99fda09c-e198-4fda-bba0-b93e100cab98_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_aba8ee51-c70b-46cc-bcd5-3beb4a03ebec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_99fda09c-e198-4fda-bba0-b93e100cab98" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_aba8ee51-c70b-46cc-bcd5-3beb4a03ebec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_HealthcareRCMBusinessMember_6dd959e1-99df-4f84-99b5-11b36996406a" xlink:href="iiiv-20250630.xsd#iiiv_HealthcareRCMBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_aba8ee51-c70b-46cc-bcd5-3beb4a03ebec" xlink:to="loc_iiiv_HealthcareRCMBusinessMember_6dd959e1-99df-4f84-99b5-11b36996406a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_MerchantServicesBusinessMember_eff86d35-1022-4188-8be6-1fd0d6a83448" xlink:href="iiiv-20250630.xsd#iiiv_MerchantServicesBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_aba8ee51-c70b-46cc-bcd5-3beb4a03ebec" xlink:to="loc_iiiv_MerchantServicesBusinessMember_eff86d35-1022-4188-8be6-1fd0d6a83448" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_6113c706-f431-4ed1-80f9-213eae7891a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_2ad76897-1c87-43d6-81be-98c6986fc527" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_6113c706-f431-4ed1-80f9-213eae7891a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_6113c706-f431-4ed1-80f9-213eae7891a6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_6113c706-f431-4ed1-80f9-213eae7891a6" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_6113c706-f431-4ed1-80f9-213eae7891a6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_1e313533-9b1c-4dcc-86cb-4fabbcd2f3c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_6113c706-f431-4ed1-80f9-213eae7891a6" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_1e313533-9b1c-4dcc-86cb-4fabbcd2f3c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember_2238c121-cf94-4653-ba44-a843401969fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_1e313533-9b1c-4dcc-86cb-4fabbcd2f3c6" xlink:to="loc_us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember_2238c121-cf94-4653-ba44-a843401969fa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue_f3ad00f7-5aad-4fea-9133-43026c7958c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue_f3ad00f7-5aad-4fea-9133-43026c7958c6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingExpenseAbstract_ba35324e-1534-4bb2-b30b-cb0c73db22d8" xlink:href="iiiv-20250630.xsd#iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:to="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingExpenseAbstract_ba35324e-1534-4bb2-b30b-cb0c73db22d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOtherCostsOfServices_06d150bd-e19f-46c6-a144-f96dc71abc2a" xlink:href="iiiv-20250630.xsd#iiiv_DisposalGroupIncludingDiscontinuedOperationOtherCostsOfServices"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingExpenseAbstract_ba35324e-1534-4bb2-b30b-cb0c73db22d8" xlink:to="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOtherCostsOfServices_06d150bd-e19f-46c6-a144-f96dc71abc2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationGeneralAndAdministrativeExpense_d35f01ce-89e8-428e-a25f-bb802cd3bd71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingExpenseAbstract_ba35324e-1534-4bb2-b30b-cb0c73db22d8" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationGeneralAndAdministrativeExpense_d35f01ce-89e8-428e-a25f-bb802cd3bd71" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationDepreciationAndAmortization_bacda9d3-4ad7-4aea-b338-5378319fb5e8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingExpenseAbstract_ba35324e-1534-4bb2-b30b-cb0c73db22d8" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationDepreciationAndAmortization_bacda9d3-4ad7-4aea-b338-5378319fb5e8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_d28b2448-71f6-4814-849b-907550d6ece9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingExpenseAbstract_ba35324e-1534-4bb2-b30b-cb0c73db22d8" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_d28b2448-71f6-4814-849b-907550d6ece9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense_6f27a1b8-9d99-4c93-b7e6-f5ccf3dec76c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOperatingExpenseAbstract_ba35324e-1534-4bb2-b30b-cb0c73db22d8" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense_6f27a1b8-9d99-4c93-b7e6-f5ccf3dec76c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingIncomeLoss_6e951944-80ec-4272-8443-1b3125527852" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingIncomeLoss_6e951944-80ec-4272-8443-1b3125527852" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestExpense_6b98c57c-0798-4dfb-aa85-a5d9cbcb957d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestExpense_6b98c57c-0798-4dfb-aa85-a5d9cbcb957d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOtherNonoperatingIncomeExpense_494e5157-346a-4f51-a4f9-5c17a7fe07c4" xlink:href="iiiv-20250630.xsd#iiiv_DisposalGroupIncludingDiscontinuedOperationOtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:to="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationOtherNonoperatingIncomeExpense_494e5157-346a-4f51-a4f9-5c17a7fe07c4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationNonoperatingIncomeExpense_e4ecab62-28c0-4f6f-8ff2-a690dde42434" xlink:href="iiiv-20250630.xsd#iiiv_DisposalGroupIncludingDiscontinuedOperationNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:to="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationNonoperatingIncomeExpense_e4ecab62-28c0-4f6f-8ff2-a690dde42434" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax_f3ab05db-a776-4bee-a6b0-844b72c6edaf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:to="loc_us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax_f3ab05db-a776-4bee-a6b0-844b72c6edaf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationTaxEffectOfIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriod_a862149b-5ca2-4a5c-ba9a-a3015566363e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationTaxEffectOfIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:to="loc_us-gaap_DiscontinuedOperationTaxEffectOfIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriod_a862149b-5ca2-4a5c-ba9a-a3015566363e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodNetOfTax_e85ff1c3-af24-4ffa-9605-a8e18cbe65f1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:to="loc_us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodNetOfTax_e85ff1c3-af24-4ffa-9605-a8e18cbe65f1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToNoncontrollingInterest_39e956b3-281c-4611-a11a-5b665c105662" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToNoncontrollingInterest_39e956b3-281c-4611-a11a-5b665c105662" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity_bd6f3c44-f87b-4986-ae7c-8a2e9adf248d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ee9407aa-1bc4-4272-b3b6-0e62b9cf6d1c" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity_bd6f3c44-f87b-4986-ae7c-8a2e9adf248d" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/DISCONTINUEDOPERATIONSCashFlowDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#DISCONTINUEDOPERATIONSCashFlowDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/DISCONTINUEDOPERATIONSCashFlowDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_ecfa5662-6072-473b-8912-6b997a135ba3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_ecfa5662-6072-473b-8912-6b997a135ba3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_c393d1f2-29e3-4dd6-92b2-bd336a3d58a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_ecfa5662-6072-473b-8912-6b997a135ba3" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_c393d1f2-29e3-4dd6-92b2-bd336a3d58a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_c393d1f2-29e3-4dd6-92b2-bd336a3d58a8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_c393d1f2-29e3-4dd6-92b2-bd336a3d58a8" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_c393d1f2-29e3-4dd6-92b2-bd336a3d58a8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_38065174-3e2c-4af4-a8ab-c899a7930637" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_c393d1f2-29e3-4dd6-92b2-bd336a3d58a8" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_38065174-3e2c-4af4-a8ab-c899a7930637" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember_d0b600cd-81a3-4436-9e43-83313634694e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_38065174-3e2c-4af4-a8ab-c899a7930637" xlink:to="loc_us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember_d0b600cd-81a3-4436-9e43-83313634694e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_4139a6e2-650d-458a-880c-98054b8b31a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_ecfa5662-6072-473b-8912-6b997a135ba3" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_4139a6e2-650d-458a-880c-98054b8b31a1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_4139a6e2-650d-458a-880c-98054b8b31a1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_4139a6e2-650d-458a-880c-98054b8b31a1" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_4139a6e2-650d-458a-880c-98054b8b31a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e35d38ab-d76c-4b2f-9b96-350d10b6bb38" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_4139a6e2-650d-458a-880c-98054b8b31a1" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e35d38ab-d76c-4b2f-9b96-350d10b6bb38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_HealthcareRCMBusinessMember_456b51f2-be4f-4376-aaea-36579f026537" xlink:href="iiiv-20250630.xsd#iiiv_HealthcareRCMBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e35d38ab-d76c-4b2f-9b96-350d10b6bb38" xlink:to="loc_iiiv_HealthcareRCMBusinessMember_456b51f2-be4f-4376-aaea-36579f026537" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_MerchantServicesBusinessMember_3b2c760e-f3b8-457f-b3cd-3f6252419bc0" xlink:href="iiiv-20250630.xsd#iiiv_MerchantServicesBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e35d38ab-d76c-4b2f-9b96-350d10b6bb38" xlink:to="loc_iiiv_MerchantServicesBusinessMember_3b2c760e-f3b8-457f-b3cd-3f6252419bc0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_7ba8f3b2-bd4b-446b-b938-7c8981c73255" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_7ba8f3b2-bd4b-446b-b938-7c8981c73255" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_ef552fd6-ec35-468c-a25f-72c1cf19526a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:to="loc_us-gaap_ShareBasedCompensation_ef552fd6-ec35-468c-a25f-72c1cf19526a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfBusiness_47392bde-e576-4fcb-a2a3-499c902a6dc0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSaleOfBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:to="loc_us-gaap_GainLossOnSaleOfBusiness_47392bde-e576-4fcb-a2a3-499c902a6dc0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_3acd4fbb-d999-4a58-9e8a-99344c25c13e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_3acd4fbb-d999-4a58-9e8a-99344c25c13e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_a7fa8f50-73da-4472-b52f-bf4b94b4fca2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_a7fa8f50-73da-4472-b52f-bf4b94b4fca2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentForContingentConsiderationLiabilityOperatingActivities_9ebdfb65-7bad-4614-921b-12792e1a6351" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentForContingentConsiderationLiabilityOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:to="loc_us-gaap_PaymentForContingentConsiderationLiabilityOperatingActivities_9ebdfb65-7bad-4614-921b-12792e1a6351" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_1ef5111b-01a7-41f1-8be1-bb3ba216bc6e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_1ef5111b-01a7-41f1-8be1-bb3ba216bc6e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForSoftware_971bf3f5-1943-4ceb-840e-5ff0a276c9fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForSoftware"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:to="loc_us-gaap_PaymentsForSoftware_971bf3f5-1943-4ceb-840e-5ff0a276c9fb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSalesOfBusinessAffiliateAndProductiveAssets_f055ab0a-cf40-4a5d-98ad-17d07f3e6810" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSalesOfBusinessAffiliateAndProductiveAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:to="loc_us-gaap_ProceedsFromSalesOfBusinessAffiliateAndProductiveAssets_f055ab0a-cf40-4a5d-98ad-17d07f3e6810" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentForContingentConsiderationLiabilityFinancingActivities_076c3cc2-8c8a-4d39-a423-ed904ede50a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentForContingentConsiderationLiabilityFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:to="loc_us-gaap_PaymentForContingentConsiderationLiabilityFinancingActivities_076c3cc2-8c8a-4d39-a423-ed904ede50a6" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireIntangibleAssets_f6baee48-63b4-4d8c-8e00-0cf30ac2f8e2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_fa26981a-5b86-4f9b-a1bc-769595659d54" xlink:to="loc_us-gaap_PaymentsToAcquireIntangibleAssets_f6baee48-63b4-4d8c-8e00-0cf30ac2f8e2" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/DISCONTINUEDOPERATIONSSignificantNoncashTransactionsDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#DISCONTINUEDOPERATIONSSignificantNoncashTransactionsDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/DISCONTINUEDOPERATIONSSignificantNoncashTransactionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_f5c73cd0-d9da-416b-93c3-3621779e91c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_517776a2-5d4d-4bda-bed1-d57d380ed961" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_f5c73cd0-d9da-416b-93c3-3621779e91c5" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_517776a2-5d4d-4bda-bed1-d57d380ed961" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_616b73e0-9142-4b1d-9781-9af1a13525b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_517776a2-5d4d-4bda-bed1-d57d380ed961" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_616b73e0-9142-4b1d-9781-9af1a13525b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_616b73e0-9142-4b1d-9781-9af1a13525b0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_616b73e0-9142-4b1d-9781-9af1a13525b0" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_616b73e0-9142-4b1d-9781-9af1a13525b0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_7fe4ee17-d08f-4e1b-986e-951e290dafd6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_616b73e0-9142-4b1d-9781-9af1a13525b0" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_7fe4ee17-d08f-4e1b-986e-951e290dafd6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember_06fff73c-7b73-4924-856b-c216da1d9a4e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_7fe4ee17-d08f-4e1b-986e-951e290dafd6" xlink:to="loc_us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember_06fff73c-7b73-4924-856b-c216da1d9a4e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_2636b58a-3f10-4b20-b347-5b92d440a038" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_517776a2-5d4d-4bda-bed1-d57d380ed961" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_2636b58a-3f10-4b20-b347-5b92d440a038" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_2636b58a-3f10-4b20-b347-5b92d440a038_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_2636b58a-3f10-4b20-b347-5b92d440a038" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_2636b58a-3f10-4b20-b347-5b92d440a038_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e8529952-d127-4c8f-9e5a-eeed1f0d663e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_2636b58a-3f10-4b20-b347-5b92d440a038" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e8529952-d127-4c8f-9e5a-eeed1f0d663e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_HealthcareRCMBusinessMember_427c65de-79d4-40db-9fd5-8903af10b9c4" xlink:href="iiiv-20250630.xsd#iiiv_HealthcareRCMBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e8529952-d127-4c8f-9e5a-eeed1f0d663e" xlink:to="loc_iiiv_HealthcareRCMBusinessMember_427c65de-79d4-40db-9fd5-8903af10b9c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_MerchantServicesBusinessMember_d5c8c7e4-4354-4669-bc80-2a1b2d71ab37" xlink:href="iiiv-20250630.xsd#iiiv_MerchantServicesBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e8529952-d127-4c8f-9e5a-eeed1f0d663e" xlink:to="loc_iiiv_MerchantServicesBusinessMember_d5c8c7e4-4354-4669-bc80-2a1b2d71ab37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_60aab8cf-0ca1-4ae1-ace9-15454d0a570f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_f5c73cd0-d9da-416b-93c3-3621779e91c5" xlink:to="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_60aab8cf-0ca1-4ae1-ace9-15454d0a570f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESAdditionalInformationDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_a4c05827-df1f-4549-8e00-938b3b5c8ddc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_5cda61ad-87a6-44ac-8954-d6624d2473d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_a4c05827-df1f-4549-8e00-938b3b5c8ddc" xlink:to="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_5cda61ad-87a6-44ac-8954-d6624d2473d7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_b1b4574c-d667-46da-b402-83d36fddb31c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_5cda61ad-87a6-44ac-8954-d6624d2473d7" xlink:to="loc_srt_RangeAxis_b1b4574c-d667-46da-b402-83d36fddb31c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_b1b4574c-d667-46da-b402-83d36fddb31c_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_b1b4574c-d667-46da-b402-83d36fddb31c" xlink:to="loc_srt_RangeMember_b1b4574c-d667-46da-b402-83d36fddb31c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_f5944088-3308-4b92-a8e9-797e555fa910" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_b1b4574c-d667-46da-b402-83d36fddb31c" xlink:to="loc_srt_RangeMember_f5944088-3308-4b92-a8e9-797e555fa910" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_dcea5050-01b0-4d34-9bca-76a5bd5f53ba" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_f5944088-3308-4b92-a8e9-797e555fa910" xlink:to="loc_srt_MinimumMember_dcea5050-01b0-4d34-9bca-76a5bd5f53ba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_6a2bdb8c-a3f9-4f84-b35f-6069e18ac080" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_f5944088-3308-4b92-a8e9-797e555fa910" xlink:to="loc_srt_MaximumMember_6a2bdb8c-a3f9-4f84-b35f-6069e18ac080" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_SettlementAssetsAndLiabilitiesTerm_b9f9bc8d-7c3b-4d7f-97b0-eccc3601f854" xlink:href="iiiv-20250630.xsd#iiiv_SettlementAssetsAndLiabilitiesTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_a4c05827-df1f-4549-8e00-938b3b5c8ddc" xlink:to="loc_iiiv_SettlementAssetsAndLiabilitiesTerm_b9f9bc8d-7c3b-4d7f-97b0-eccc3601f854" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SettlementAssetsCurrent_de4461f2-f9a3-4f78-841a-e44f3d3cc413" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SettlementAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_a4c05827-df1f-4549-8e00-938b3b5c8ddc" xlink:to="loc_us-gaap_SettlementAssetsCurrent_de4461f2-f9a3-4f78-841a-e44f3d3cc413" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SettlementLiabilitiesCurrent_21e7ef59-56e3-4233-a7e9-c44da64e1138" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SettlementLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_a4c05827-df1f-4549-8e00-938b3b5c8ddc" xlink:to="loc_us-gaap_SettlementLiabilitiesCurrent_21e7ef59-56e3-4233-a7e9-c44da64e1138" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryNet_eb99af56-8ef5-4a6b-89d2-9d82fdb75a07" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_a4c05827-df1f-4549-8e00-938b3b5c8ddc" xlink:to="loc_us-gaap_InventoryNet_eb99af56-8ef5-4a6b-89d2-9d82fdb75a07" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetGross_a6e980ef-63ec-42a4-aedf-ffd7ff7eb9ba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerAssetGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_a4c05827-df1f-4549-8e00-938b3b5c8ddc" xlink:to="loc_us-gaap_ContractWithCustomerAssetGross_a6e980ef-63ec-42a4-aedf-ffd7ff7eb9ba" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostGross_33bf5c53-1f50-4446-9e82-d2bcd9748e53" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedContractCostGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_a4c05827-df1f-4549-8e00-938b3b5c8ddc" xlink:to="loc_us-gaap_CapitalizedContractCostGross_33bf5c53-1f50-4446-9e82-d2bcd9748e53" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostAmortization_b95dcb26-ce53-4383-b07e-49635acfd833" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedContractCostAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_a4c05827-df1f-4549-8e00-938b3b5c8ddc" xlink:to="loc_us-gaap_CapitalizedContractCostAmortization_b95dcb26-ce53-4383-b07e-49635acfd833" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESOperatingExpensesorEarningsPerShareDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESOperatingExpensesorEarningsPerShareDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESOperatingExpensesorEarningsPerShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationLineItems_c149766d-538f-4d52-95cc-5bb7dbb888cb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReclassificationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationTable_fed30f93-1969-4ff5-87f7-327199b28c90" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReclassificationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ReclassificationLineItems_c149766d-538f-4d52-95cc-5bb7dbb888cb" xlink:to="loc_us-gaap_ReclassificationTable_fed30f93-1969-4ff5-87f7-327199b28c90" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAxis_6e791755-facc-49a2-962a-c3764676efdf" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationTable_fed30f93-1969-4ff5-87f7-327199b28c90" xlink:to="loc_srt_RestatementAxis_6e791755-facc-49a2-962a-c3764676efdf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_6e791755-facc-49a2-962a-c3764676efdf_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_6e791755-facc-49a2-962a-c3764676efdf" xlink:to="loc_srt_RestatementDomain_6e791755-facc-49a2-962a-c3764676efdf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_f52da9b3-1ee2-42e6-ab49-cc1f123bbaae" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_6e791755-facc-49a2-962a-c3764676efdf" xlink:to="loc_srt_RestatementDomain_f52da9b3-1ee2-42e6-ab49-cc1f123bbaae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioPreviouslyReportedMember_caf133d2-eed8-4d2f-a271-66cb62b46432" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioPreviouslyReportedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_f52da9b3-1ee2-42e6-ab49-cc1f123bbaae" xlink:to="loc_srt_ScenarioPreviouslyReportedMember_caf133d2-eed8-4d2f-a271-66cb62b46432" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAdjustmentMember_d1d31a76-9e67-43e9-b03f-3dc44ed1f4f1" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_f52da9b3-1ee2-42e6-ab49-cc1f123bbaae" xlink:to="loc_srt_RestatementAdjustmentMember_d1d31a76-9e67-43e9-b03f-3dc44ed1f4f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCostOfOperatingRevenue_1e27d794-91fe-4255-b920-f57d901ca721" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCostOfOperatingRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationLineItems_c149766d-538f-4d52-95cc-5bb7dbb888cb" xlink:to="loc_us-gaap_OtherCostOfOperatingRevenue_1e27d794-91fe-4255-b920-f57d901ca721" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_58caa4ab-645a-4efd-b495-f0cddb88f817" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationLineItems_c149766d-538f-4d52-95cc-5bb7dbb888cb" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_58caa4ab-645a-4efd-b495-f0cddb88f817" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESDisaggregatedRevenueDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESDisaggregatedRevenueDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESDisaggregatedRevenueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_5f8eac77-35fe-40b2-a20b-1bd04e544398" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_f60021d5-8de5-4a69-89c1-1a0c83659427" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_5f8eac77-35fe-40b2-a20b-1bd04e544398" xlink:to="loc_us-gaap_ConcentrationRiskTable_f60021d5-8de5-4a69-89c1-1a0c83659427" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_c9a3ec0c-6305-432d-a795-8ae862c00f3e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_f60021d5-8de5-4a69-89c1-1a0c83659427" xlink:to="loc_srt_ProductOrServiceAxis_c9a3ec0c-6305-432d-a795-8ae862c00f3e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_c9a3ec0c-6305-432d-a795-8ae862c00f3e_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_c9a3ec0c-6305-432d-a795-8ae862c00f3e" xlink:to="loc_srt_ProductsAndServicesDomain_c9a3ec0c-6305-432d-a795-8ae862c00f3e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_8ed082e1-5e23-4069-ab41-dd8242566edc" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_c9a3ec0c-6305-432d-a795-8ae862c00f3e" xlink:to="loc_srt_ProductsAndServicesDomain_8ed082e1-5e23-4069-ab41-dd8242566edc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LicenseAndServiceMember_c5012be3-09e6-4af4-b79d-fe6c7c873841" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LicenseAndServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8ed082e1-5e23-4069-ab41-dd8242566edc" xlink:to="loc_us-gaap_LicenseAndServiceMember_c5012be3-09e6-4af4-b79d-fe6c7c873841" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_ProprietaryPaymentsRevenueMember_39b94ded-f179-47e1-9581-c864f6bc6b9c" xlink:href="iiiv-20250630.xsd#iiiv_ProprietaryPaymentsRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8ed082e1-5e23-4069-ab41-dd8242566edc" xlink:to="loc_iiiv_ProprietaryPaymentsRevenueMember_39b94ded-f179-47e1-9581-c864f6bc6b9c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_OtherRevenueMember_e02d6e0f-be1e-4153-bb93-ae12e89723b2" xlink:href="iiiv-20250630.xsd#iiiv_OtherRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8ed082e1-5e23-4069-ab41-dd8242566edc" xlink:to="loc_iiiv_OtherRevenueMember_e02d6e0f-be1e-4153-bb93-ae12e89723b2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_d9c29c1c-3653-4a10-9054-5c939c8078c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_f60021d5-8de5-4a69-89c1-1a0c83659427" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_d9c29c1c-3653-4a10-9054-5c939c8078c5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_d9c29c1c-3653-4a10-9054-5c939c8078c5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_d9c29c1c-3653-4a10-9054-5c939c8078c5" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_d9c29c1c-3653-4a10-9054-5c939c8078c5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_3acf92f8-cba2-458c-af68-cc3a9965ecff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_d9c29c1c-3653-4a10-9054-5c939c8078c5" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_3acf92f8-cba2-458c-af68-cc3a9965ecff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredOverTimeMember_68d11310-3890-4aee-89ec-48056c559249" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferredOverTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_3acf92f8-cba2-458c-af68-cc3a9965ecff" xlink:to="loc_us-gaap_TransferredOverTimeMember_68d11310-3890-4aee-89ec-48056c559249" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredAtPointInTimeMember_dc6ada47-eed0-49ed-ab9b-45e2f2fd8263" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferredAtPointInTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_3acf92f8-cba2-458c-af68-cc3a9965ecff" xlink:to="loc_us-gaap_TransferredAtPointInTimeMember_dc6ada47-eed0-49ed-ab9b-45e2f2fd8263" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_fedd9fbe-bbf1-42cd-836b-3edd2fcdbc59" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_5f8eac77-35fe-40b2-a20b-1bd04e544398" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_fedd9fbe-bbf1-42cd-836b-3edd2fcdbc59" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/ACQUISITIONSAdditionalInformationDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#ACQUISITIONSAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/ACQUISITIONSAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_b52b2976-eac4-42b4-a421-d3d6842468ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_b52b2976-eac4-42b4-a421-d3d6842468ac" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_3775b3dc-723c-4766-aeff-f6fabad3237a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_b52b2976-eac4-42b4-a421-d3d6842468ac" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_3775b3dc-723c-4766-aeff-f6fabad3237a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_3775b3dc-723c-4766-aeff-f6fabad3237a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_3775b3dc-723c-4766-aeff-f6fabad3237a" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_3775b3dc-723c-4766-aeff-f6fabad3237a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9970c370-93ca-483b-b8b1-5b0508b048de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_3775b3dc-723c-4766-aeff-f6fabad3237a" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9970c370-93ca-483b-b8b1-5b0508b048de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_UtilityBillingSoftwareCompanyMember_8de7f7bc-6aed-4c92-9803-276ff004ebf6" xlink:href="iiiv-20250630.xsd#iiiv_UtilityBillingSoftwareCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9970c370-93ca-483b-b8b1-5b0508b048de" xlink:to="loc_iiiv_UtilityBillingSoftwareCompanyMember_8de7f7bc-6aed-4c92-9803-276ff004ebf6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_SoftwareBusinessAcquisitionMember_00a86a25-0cd6-428b-b50c-5657845dd2e1" xlink:href="iiiv-20250630.xsd#iiiv_SoftwareBusinessAcquisitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9970c370-93ca-483b-b8b1-5b0508b048de" xlink:to="loc_iiiv_SoftwareBusinessAcquisitionMember_00a86a25-0cd6-428b-b50c-5657845dd2e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_EdulokaLtd.Member_53dd3658-b806-402a-b46f-74479b659f67" xlink:href="iiiv-20250630.xsd#iiiv_EdulokaLtd.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9970c370-93ca-483b-b8b1-5b0508b048de" xlink:to="loc_iiiv_EdulokaLtd.Member_53dd3658-b806-402a-b46f-74479b659f67" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_9a770aef-0980-4e53-9db9-5de903290686" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_b52b2976-eac4-42b4-a421-d3d6842468ac" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_9a770aef-0980-4e53-9db9-5de903290686" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_9a770aef-0980-4e53-9db9-5de903290686_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_9a770aef-0980-4e53-9db9-5de903290686" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_9a770aef-0980-4e53-9db9-5de903290686_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_e0515801-593a-4589-94de-e4c0958796f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_9a770aef-0980-4e53-9db9-5de903290686" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_e0515801-593a-4589-94de-e4c0958796f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_c9c2c1b9-b468-4984-b252-7f5a7d146f88" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_e0515801-593a-4589-94de-e4c0958796f3" xlink:to="loc_us-gaap_CustomerRelationshipsMember_c9c2c1b9-b468-4984-b252-7f5a7d146f88" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_0fe07e11-a72e-4d2e-984f-4d996604bcd3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_e0515801-593a-4589-94de-e4c0958796f3" xlink:to="loc_us-gaap_TradeNamesMember_0fe07e11-a72e-4d2e-984f-4d996604bcd3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerSoftwareIntangibleAssetMember_e0c1605f-ec7e-4e36-bf2f-d4c1aa023849" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerSoftwareIntangibleAssetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_e0515801-593a-4589-94de-e4c0958796f3" xlink:to="loc_us-gaap_ComputerSoftwareIntangibleAssetMember_e0c1605f-ec7e-4e36-bf2f-d4c1aa023849" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_a73eb1fc-1a38-48c8-ab62-d96d8a86a591" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_b52b2976-eac4-42b4-a421-d3d6842468ac" xlink:to="loc_us-gaap_StatementClassOfStockAxis_a73eb1fc-1a38-48c8-ab62-d96d8a86a591" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_a73eb1fc-1a38-48c8-ab62-d96d8a86a591_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_a73eb1fc-1a38-48c8-ab62-d96d8a86a591" xlink:to="loc_us-gaap_ClassOfStockDomain_a73eb1fc-1a38-48c8-ab62-d96d8a86a591_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_096e1fa0-1f04-4225-8963-740d7139e93c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_a73eb1fc-1a38-48c8-ab62-d96d8a86a591" xlink:to="loc_us-gaap_ClassOfStockDomain_096e1fa0-1f04-4225-8963-740d7139e93c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_81e159f3-799a-41fb-a8f3-edb217421639" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_096e1fa0-1f04-4225-8963-740d7139e93c" xlink:to="loc_us-gaap_CommonClassAMember_81e159f3-799a-41fb-a8f3-edb217421639" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_098c8476-1422-4365-857a-cf632f9458b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_098c8476-1422-4365-857a-cf632f9458b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_bfcc664e-7e21-4f1e-a02f-d0e0e8bd7bf4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_bfcc664e-7e21-4f1e-a02f-d0e0e8bd7bf4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_734b82ec-4b30-40a0-979f-af26960178f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_734b82ec-4b30-40a0-979f-af26960178f5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_fe00373e-035e-478f-ba8f-25d6dad5761e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_fe00373e-035e-478f-ba8f-25d6dad5761e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_07af5493-e529-47df-bb71-119f8afc8fd1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_07af5493-e529-47df-bb71-119f8afc8fd1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_a4980451-8901-4849-98e8-8637c48d0e72" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_a4980451-8901-4849-98e8-8637c48d0e72" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_0a4f8bc1-755a-4426-a0d3-a0f2be1527bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_0a4f8bc1-755a-4426-a0d3-a0f2be1527bf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_7112bb99-02a9-41e0-8f1c-7d3a5f7c8203" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_7112bb99-02a9-41e0-8f1c-7d3a5f7c8203" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_2f788e45-613c-44c0-9f03-00b86b27ac0f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:to="loc_us-gaap_Goodwill_2f788e45-613c-44c0-9f03-00b86b27ac0f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_4e7b228f-4421-497e-ac67-56ba75794e2a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:to="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_4e7b228f-4421-497e-ac67-56ba75794e2a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_fdbeb425-bf76-4af5-9f02-60b1b5a062ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_b4aabd4e-f1ac-4fd5-83f7-8ff42aaa4c6a" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_fdbeb425-bf76-4af5-9f02-60b1b5a062ff" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/ACQUISITIONSPreliminaryPurchaseConsiderationoftheAcquiredAssetsandAssumedLiabilitiesDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#ACQUISITIONSPreliminaryPurchaseConsiderationoftheAcquiredAssetsandAssumedLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/ACQUISITIONSPreliminaryPurchaseConsiderationoftheAcquiredAssetsandAssumedLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_48a550fc-041c-48df-babd-d5336143e93e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_5084f92b-db86-4897-bbc6-b5a903f61547" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_48a550fc-041c-48df-babd-d5336143e93e" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_5084f92b-db86-4897-bbc6-b5a903f61547" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_1c4caf65-c927-48ea-8c28-b1bae16c4c07" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_5084f92b-db86-4897-bbc6-b5a903f61547" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_1c4caf65-c927-48ea-8c28-b1bae16c4c07" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_1c4caf65-c927-48ea-8c28-b1bae16c4c07_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_1c4caf65-c927-48ea-8c28-b1bae16c4c07" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_1c4caf65-c927-48ea-8c28-b1bae16c4c07_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_bc1842e8-7693-4fc9-a030-d596ea351101" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_1c4caf65-c927-48ea-8c28-b1bae16c4c07" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_bc1842e8-7693-4fc9-a030-d596ea351101" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_UtilityBillingSoftwareCompanyMember_6eb46791-4eab-441e-9dbe-7b1392d72e78" xlink:href="iiiv-20250630.xsd#iiiv_UtilityBillingSoftwareCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_bc1842e8-7693-4fc9-a030-d596ea351101" xlink:to="loc_iiiv_UtilityBillingSoftwareCompanyMember_6eb46791-4eab-441e-9dbe-7b1392d72e78" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_EdulokaLtd.Member_3526f41e-8968-4169-9926-e7942dbe8f9c" xlink:href="iiiv-20250630.xsd#iiiv_EdulokaLtd.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_bc1842e8-7693-4fc9-a030-d596ea351101" xlink:to="loc_iiiv_EdulokaLtd.Member_3526f41e-8968-4169-9926-e7942dbe8f9c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_965baf2b-540f-403a-9ef8-dbcea826824b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_5084f92b-db86-4897-bbc6-b5a903f61547" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_965baf2b-540f-403a-9ef8-dbcea826824b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_965baf2b-540f-403a-9ef8-dbcea826824b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_965baf2b-540f-403a-9ef8-dbcea826824b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_965baf2b-540f-403a-9ef8-dbcea826824b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_32276e0b-6f46-4fca-867f-ebfde1bf9ea8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_965baf2b-540f-403a-9ef8-dbcea826824b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_32276e0b-6f46-4fca-867f-ebfde1bf9ea8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerSoftwareIntangibleAssetMember_fc30f42b-8ef5-4dbe-8310-b0e9fe45d646" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerSoftwareIntangibleAssetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_32276e0b-6f46-4fca-867f-ebfde1bf9ea8" xlink:to="loc_us-gaap_ComputerSoftwareIntangibleAssetMember_fc30f42b-8ef5-4dbe-8310-b0e9fe45d646" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_fffeb7f2-041b-4047-9ae8-4aee70363253" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_32276e0b-6f46-4fca-867f-ebfde1bf9ea8" xlink:to="loc_us-gaap_CustomerRelationshipsMember_fffeb7f2-041b-4047-9ae8-4aee70363253" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_832d812a-6b49-491d-aa6d-42d374c03b0d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_32276e0b-6f46-4fca-867f-ebfde1bf9ea8" xlink:to="loc_us-gaap_TradeNamesMember_832d812a-6b49-491d-aa6d-42d374c03b0d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_71b4a3f9-3b95-41e0-b928-ad1ac0030749" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_48a550fc-041c-48df-babd-d5336143e93e" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_71b4a3f9-3b95-41e0-b928-ad1ac0030749" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_d8418b99-6423-4449-8a6f-ccfc53133b83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_48a550fc-041c-48df-babd-d5336143e93e" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_d8418b99-6423-4449-8a6f-ccfc53133b83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_80bd4e34-76ad-4284-bea1-f0095259f3fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_48a550fc-041c-48df-babd-d5336143e93e" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_80bd4e34-76ad-4284-bea1-f0095259f3fe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_67c0b945-0a4c-45b3-8e0f-df2dbd7b30bd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_48a550fc-041c-48df-babd-d5336143e93e" xlink:to="loc_us-gaap_Goodwill_67c0b945-0a4c-45b3-8e0f-df2dbd7b30bd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill_1451dde7-2a31-41d4-a87e-26ee00ea3616" xlink:href="iiiv-20250630.xsd#iiiv_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_48a550fc-041c-48df-babd-d5336143e93e" xlink:to="loc_iiiv_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill_1451dde7-2a31-41d4-a87e-26ee00ea3616" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccruedExpenses_cd171d88-129a-40db-9601-8e202c15d9f3" xlink:href="iiiv-20250630.xsd#iiiv_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccruedExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_48a550fc-041c-48df-babd-d5336143e93e" xlink:to="loc_iiiv_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccruedExpenses_cd171d88-129a-40db-9601-8e202c15d9f3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesDeferredRevenue_b7d451a3-0a6c-4f11-b1eb-679697487d31" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesDeferredRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_48a550fc-041c-48df-babd-d5336143e93e" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesDeferredRevenue_b7d451a3-0a6c-4f11-b1eb-679697487d31" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_11fb20c6-758b-4377-9518-9ebe21b24432" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_48a550fc-041c-48df-babd-d5336143e93e" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_11fb20c6-758b-4377-9518-9ebe21b24432" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/PREPAIDEXPENSESANDOTHERCURRENTASSETSDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#PREPAIDEXPENSESANDOTHERCURRENTASSETSDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/PREPAIDEXPENSESANDOTHERCURRENTASSETSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureLineItems_e289590f-08ec-4b84-8c36-acd0f2a26643" xlink:href="iiiv-20250630.xsd#iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureTable_460ca57a-09a9-4b56-9963-cddf4a9c8f9d" xlink:href="iiiv-20250630.xsd#iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureLineItems_e289590f-08ec-4b84-8c36-acd0f2a26643" xlink:to="loc_iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureTable_460ca57a-09a9-4b56-9963-cddf4a9c8f9d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_6e952dc5-eb08-4752-bf0c-06ce185fc749" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureTable_460ca57a-09a9-4b56-9963-cddf4a9c8f9d" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_6e952dc5-eb08-4752-bf0c-06ce185fc749" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_6e952dc5-eb08-4752-bf0c-06ce185fc749_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_6e952dc5-eb08-4752-bf0c-06ce185fc749" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_6e952dc5-eb08-4752-bf0c-06ce185fc749_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_4f746cb0-629b-4306-9f4f-99325e711a01" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_6e952dc5-eb08-4752-bf0c-06ce185fc749" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_4f746cb0-629b-4306-9f4f-99325e711a01" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_HealthcareRCMBusinessMember_3f8ffb18-3f6c-4291-aba5-25304348fac5" xlink:href="iiiv-20250630.xsd#iiiv_HealthcareRCMBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_4f746cb0-629b-4306-9f4f-99325e711a01" xlink:to="loc_iiiv_HealthcareRCMBusinessMember_3f8ffb18-3f6c-4291-aba5-25304348fac5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_PrepaidInventory_5d3bf7d6-4713-404b-b185-fece6406c7e8" xlink:href="iiiv-20250630.xsd#iiiv_PrepaidInventory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureLineItems_e289590f-08ec-4b84-8c36-acd0f2a26643" xlink:to="loc_iiiv_PrepaidInventory_5d3bf7d6-4713-404b-b185-fece6406c7e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_PrepaidLicenses_19602bed-3ee4-4877-a3a6-854483fd9331" xlink:href="iiiv-20250630.xsd#iiiv_PrepaidLicenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureLineItems_e289590f-08ec-4b84-8c36-acd0f2a26643" xlink:to="loc_iiiv_PrepaidLicenses_19602bed-3ee4-4877-a3a6-854483fd9331" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidInsurance_95360493-7d15-4e5c-9227-4ce6b6114e1c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidInsurance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureLineItems_e289590f-08ec-4b84-8c36-acd0f2a26643" xlink:to="loc_us-gaap_PrepaidInsurance_95360493-7d15-4e5c-9227-4ce6b6114e1c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivablesNetCurrent_47d666da-1cb5-40f6-8a7a-a6be053667f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReceivablesNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureLineItems_e289590f-08ec-4b84-8c36-acd0f2a26643" xlink:to="loc_us-gaap_ReceivablesNetCurrent_47d666da-1cb5-40f6-8a7a-a6be053667f5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsCurrent_f980dc66-d5f6-466f-9628-0383b555df83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureLineItems_e289590f-08ec-4b84-8c36-acd0f2a26643" xlink:to="loc_us-gaap_OtherAssetsCurrent_f980dc66-d5f6-466f-9628-0383b555df83" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_49256d43-8803-4186-b5e3-e9ad71771ce9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureLineItems_e289590f-08ec-4b84-8c36-acd0f2a26643" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_49256d43-8803-4186-b5e3-e9ad71771ce9" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/GOODWILLANDINTANGIBLEASSETSChangesinGoodwillDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#GOODWILLANDINTANGIBLEASSETSChangesinGoodwillDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/GOODWILLANDINTANGIBLEASSETSChangesinGoodwillDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_b44b7408-f73c-47f8-b5f8-e00ea32ecab5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_de8d0c2b-b9b2-4ac3-b311-52b531927bf9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_b44b7408-f73c-47f8-b5f8-e00ea32ecab5" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_de8d0c2b-b9b2-4ac3-b311-52b531927bf9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_00675f7d-423d-4477-b0e7-f52f3b5d253f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_de8d0c2b-b9b2-4ac3-b311-52b531927bf9" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_00675f7d-423d-4477-b0e7-f52f3b5d253f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_00675f7d-423d-4477-b0e7-f52f3b5d253f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_00675f7d-423d-4477-b0e7-f52f3b5d253f" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_00675f7d-423d-4477-b0e7-f52f3b5d253f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_66f5ab2f-feed-4b38-84c0-7fc445690ffc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_00675f7d-423d-4477-b0e7-f52f3b5d253f" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_66f5ab2f-feed-4b38-84c0-7fc445690ffc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_HealthcareRCMBusinessMember_9fc3a4ac-c21f-49de-8aff-2cca0000a55b" xlink:href="iiiv-20250630.xsd#iiiv_HealthcareRCMBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_66f5ab2f-feed-4b38-84c0-7fc445690ffc" xlink:to="loc_iiiv_HealthcareRCMBusinessMember_9fc3a4ac-c21f-49de-8aff-2cca0000a55b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillRollForward_27407aff-5154-4472-afe8-ac81a424a4d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_b44b7408-f73c-47f8-b5f8-e00ea32ecab5" xlink:to="loc_us-gaap_GoodwillRollForward_27407aff-5154-4472-afe8-ac81a424a4d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_8b9a1d9b-edda-4453-a582-a03031be8c0a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_27407aff-5154-4472-afe8-ac81a424a4d1" xlink:to="loc_us-gaap_Goodwill_8b9a1d9b-edda-4453-a582-a03031be8c0a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_9fac3c00-6fae-4b3f-b378-ab3bbb53584f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillPurchaseAccountingAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_27407aff-5154-4472-afe8-ac81a424a4d1" xlink:to="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_9fac3c00-6fae-4b3f-b378-ab3bbb53584f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_fcee1b04-2a1b-451d-9def-16b8cd6e57da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationGoodwillNoncurrent_7570b36a-7295-4245-9ffc-a9a1eed68a18" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationGoodwillNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_27407aff-5154-4472-afe8-ac81a424a4d1" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationGoodwillNoncurrent_7570b36a-7295-4245-9ffc-a9a1eed68a18" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/GOODWILLANDINTANGIBLEASSETSIntangibleAssetsDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#GOODWILLANDINTANGIBLEASSETSIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/GOODWILLANDINTANGIBLEASSETSIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_1cf35cf6-6a46-439a-bc35-08de2c14e172" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_98b87546-f5d8-4836-9a3d-ed8314b62369" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_1cf35cf6-6a46-439a-bc35-08de2c14e172" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_98b87546-f5d8-4836-9a3d-ed8314b62369" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_bcec7cac-11c8-4b88-82d8-bd4f685caa68" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_98b87546-f5d8-4836-9a3d-ed8314b62369" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_bcec7cac-11c8-4b88-82d8-bd4f685caa68" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_bcec7cac-11c8-4b88-82d8-bd4f685caa68_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_bcec7cac-11c8-4b88-82d8-bd4f685caa68" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_bcec7cac-11c8-4b88-82d8-bd4f685caa68_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_26ed4e88-ea69-4ea7-9945-e84b3335c27f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_bcec7cac-11c8-4b88-82d8-bd4f685caa68" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_26ed4e88-ea69-4ea7-9945-e84b3335c27f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_9c4f1ce7-99cb-419a-b6b6-d1de5da171cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_26ed4e88-ea69-4ea7-9945-e84b3335c27f" xlink:to="loc_us-gaap_CustomerRelationshipsMember_9c4f1ce7-99cb-419a-b6b6-d1de5da171cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_0ff5d196-abff-4f5d-80b6-4d0d5bd7f767" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_26ed4e88-ea69-4ea7-9945-e84b3335c27f" xlink:to="loc_us-gaap_TradeNamesMember_0ff5d196-abff-4f5d-80b6-4d0d5bd7f767" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_NonCompeteAgreementsAndOtherIntangibleAssetsMember_b07b2971-4d4f-44c2-93ab-c1649177c8c9" xlink:href="iiiv-20250630.xsd#iiiv_NonCompeteAgreementsAndOtherIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_26ed4e88-ea69-4ea7-9945-e84b3335c27f" xlink:to="loc_iiiv_NonCompeteAgreementsAndOtherIntangibleAssetsMember_b07b2971-4d4f-44c2-93ab-c1649177c8c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_59a3c8c2-2489-4680-84d4-4479ea106db9" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_98b87546-f5d8-4836-9a3d-ed8314b62369" xlink:to="loc_srt_RangeAxis_59a3c8c2-2489-4680-84d4-4479ea106db9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_59a3c8c2-2489-4680-84d4-4479ea106db9_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_59a3c8c2-2489-4680-84d4-4479ea106db9" xlink:to="loc_srt_RangeMember_59a3c8c2-2489-4680-84d4-4479ea106db9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_0b5d4f66-170d-4b8f-9cfb-d52c8c77ce5f" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_59a3c8c2-2489-4680-84d4-4479ea106db9" xlink:to="loc_srt_RangeMember_0b5d4f66-170d-4b8f-9cfb-d52c8c77ce5f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_bfab92e7-e751-4146-9c78-947a5e4b68bf" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_0b5d4f66-170d-4b8f-9cfb-d52c8c77ce5f" xlink:to="loc_srt_MinimumMember_bfab92e7-e751-4146-9c78-947a5e4b68bf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_22d8beb0-319c-4621-86a6-56e19cebee58" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_0b5d4f66-170d-4b8f-9cfb-d52c8c77ce5f" xlink:to="loc_srt_MaximumMember_22d8beb0-319c-4621-86a6-56e19cebee58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_45c31909-9ca0-4f25-90df-11dace3ff4d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_1cf35cf6-6a46-439a-bc35-08de2c14e172" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_45c31909-9ca0-4f25-90df-11dace3ff4d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_341e251e-3370-42a4-a125-761849723fb7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_1cf35cf6-6a46-439a-bc35-08de2c14e172" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_341e251e-3370-42a4-a125-761849723fb7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_be57d794-3db5-4cc2-800f-3a84b397f7cf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_1cf35cf6-6a46-439a-bc35-08de2c14e172" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_be57d794-3db5-4cc2-800f-3a84b397f7cf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_74af3c5d-241c-4f5b-ace7-b9423e4506da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_1cf35cf6-6a46-439a-bc35-08de2c14e172" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_74af3c5d-241c-4f5b-ace7-b9423e4506da" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_1248efb9-bf4b-4ccf-b29f-e202eadf4518" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_cb8b7119-d3c8-4bdf-b9d5-2bf840a7c52d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_1248efb9-bf4b-4ccf-b29f-e202eadf4518" xlink:to="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_cb8b7119-d3c8-4bdf-b9d5-2bf840a7c52d" xlink:type="arc" order="2" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_b38e4f6a-d321-495f-bfae-50669b93cdc1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_cb8b7119-d3c8-4bdf-b9d5-2bf840a7c52d" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_b38e4f6a-d321-495f-bfae-50669b93cdc1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_b38e4f6a-d321-495f-bfae-50669b93cdc1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_b38e4f6a-d321-495f-bfae-50669b93cdc1" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_b38e4f6a-d321-495f-bfae-50669b93cdc1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_a66db1a7-389c-4f1f-a493-07688a83ce11" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_b38e4f6a-d321-495f-bfae-50669b93cdc1" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_a66db1a7-389c-4f1f-a493-07688a83ce11" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksMember_f52e200b-0c3f-45ae-9c24-04c38fb1baa6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TrademarksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_a66db1a7-389c-4f1f-a493-07688a83ce11" xlink:to="loc_us-gaap_TrademarksMember_f52e200b-0c3f-45ae-9c24-04c38fb1baa6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_05fb12ef-681f-4ca4-a9a4-dfbf071ee61a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_1248efb9-bf4b-4ccf-b29f-e202eadf4518" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_05fb12ef-681f-4ca4-a9a4-dfbf071ee61a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/ACCRUEDEXPENSESANDOTHERLIABILITIESDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#ACCRUEDEXPENSESANDOTHERLIABILITIESDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/ACCRUEDEXPENSESANDOTHERLIABILITIESDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_ed24ba8a-9a0b-428c-8dd2-5940eca70eb8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_ed24ba8a-9a0b-428c-8dd2-5940eca70eb8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_32bfebcd-ed82-401f-a3e6-30d03d889e92" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_ed24ba8a-9a0b-428c-8dd2-5940eca70eb8" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_32bfebcd-ed82-401f-a3e6-30d03d889e92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_32bfebcd-ed82-401f-a3e6-30d03d889e92_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_32bfebcd-ed82-401f-a3e6-30d03d889e92" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_32bfebcd-ed82-401f-a3e6-30d03d889e92_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_5b92ad58-4221-4f0b-b234-8733c1e6e8c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_32bfebcd-ed82-401f-a3e6-30d03d889e92" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_5b92ad58-4221-4f0b-b234-8733c1e6e8c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_MerchantServicesBusinessMember_ef4681be-7d83-46d2-9136-ab4cc5777c31" xlink:href="iiiv-20250630.xsd#iiiv_MerchantServicesBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_5b92ad58-4221-4f0b-b234-8733c1e6e8c1" xlink:to="loc_iiiv_MerchantServicesBusinessMember_ef4681be-7d83-46d2-9136-ab4cc5777c31" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_HealthcareRCMBusinessMember_e5ff823f-44c8-4591-96fb-2a39d7bdea33" xlink:href="iiiv-20250630.xsd#iiiv_HealthcareRCMBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_5b92ad58-4221-4f0b-b234-8733c1e6e8c1" xlink:to="loc_iiiv_HealthcareRCMBusinessMember_e5ff823f-44c8-4591-96fb-2a39d7bdea33" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_e66f9dc2-330e-4007-9872-8981909070c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_e66f9dc2-330e-4007-9872-8981909070c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPayableCurrent_2764ae6a-caea-4400-b5b8-5f020fc147ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_us-gaap_InterestPayableCurrent_2764ae6a-caea-4400-b5b8-5f020fc147ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityCurrent_c8c0fc81-471c-4920-a785-aea8eadf754a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationContingentConsiderationLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityCurrent_c8c0fc81-471c-4920-a785-aea8eadf754a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_EscrowLiabilitiesCurrent_d5f18d2e-e3c8-488c-a040-0fb98a7f464d" xlink:href="iiiv-20250630.xsd#iiiv_EscrowLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_iiiv_EscrowLiabilitiesCurrent_d5f18d2e-e3c8-488c-a040-0fb98a7f464d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AccruedTaxDistributionsCurrent_7b3d514f-de2c-4ce8-989c-916b009b0f75" xlink:href="iiiv-20250630.xsd#iiiv_AccruedTaxDistributionsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_iiiv_AccruedTaxDistributionsCurrent_7b3d514f-de2c-4ce8-989c-916b009b0f75" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedIncomeTaxesCurrent_4967fcca-7a5d-41aa-87d6-d4f1d60463af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedIncomeTaxesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_us-gaap_AccruedIncomeTaxesCurrent_4967fcca-7a5d-41aa-87d6-d4f1d60463af" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_TaxReceivableAgreementLiabilityCurrent_4ad76a66-2822-41a1-b70d-4907a2cef41e" xlink:href="iiiv-20250630.xsd#iiiv_TaxReceivableAgreementLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_iiiv_TaxReceivableAgreementLiabilityCurrent_4ad76a66-2822-41a1-b70d-4907a2cef41e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerRefundLiabilityCurrent_43dca472-d43b-4b6d-9552-f3fcb1f821cb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerRefundLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_us-gaap_ContractWithCustomerRefundLiabilityCurrent_43dca472-d43b-4b6d-9552-f3fcb1f821cb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SelfInsuranceReserveCurrent_7b58b633-d53c-429e-ae5e-3cc825a1ea91" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SelfInsuranceReserveCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_us-gaap_SelfInsuranceReserveCurrent_7b58b633-d53c-429e-ae5e-3cc825a1ea91" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AccruedLicenseExpensesCurrent_e51d5f85-d94d-42d3-a21f-69b8cc1218c2" xlink:href="iiiv-20250630.xsd#iiiv_AccruedLicenseExpensesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_iiiv_AccruedLicenseExpensesCurrent_e51d5f85-d94d-42d3-a21f-69b8cc1218c2" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AccruedInterchangeCurrent_bc34bed0-5ed4-458e-897d-802fe412eeea" xlink:href="iiiv-20250630.xsd#iiiv_AccruedInterchangeCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_iiiv_AccruedInterchangeCurrent_bc34bed0-5ed4-458e-897d-802fe412eeea" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_OtherAccruedLiabilitiesRelatedToDivestituresCurrent_ce91f70e-e7de-43a9-aede-917117b85e09" xlink:href="iiiv-20250630.xsd#iiiv_OtherAccruedLiabilitiesRelatedToDivestituresCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_iiiv_OtherAccruedLiabilitiesRelatedToDivestituresCurrent_ce91f70e-e7de-43a9-aede-917117b85e09" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAccruedLiabilitiesCurrent_8f76b289-2f75-40c2-bf57-158d049e051f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_us-gaap_OtherAccruedLiabilitiesCurrent_8f76b289-2f75-40c2-bf57-158d049e051f" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AccruedLiabilitiesOwedToSellersCurrent_05d75681-d1a8-46ba-8b12-58abdd2f248c" xlink:href="iiiv-20250630.xsd#iiiv_AccruedLiabilitiesOwedToSellersCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_iiiv_AccruedLiabilitiesOwedToSellersCurrent_05d75681-d1a8-46ba-8b12-58abdd2f248c" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent_34197ff6-b83c-4fbf-9b50-00879cd532b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_52dbd63d-4ec4-4713-8e12-b3308b814bea" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent_34197ff6-b83c-4fbf-9b50-00879cd532b3" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/LONGTERMDEBTNETScheduleofLongtermDebtDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#LONGTERMDEBTNETScheduleofLongtermDebtDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/LONGTERMDEBTNETScheduleofLongtermDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_f01993f6-8a28-4c98-99ec-f0183315e229" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_1559d47f-dbc3-45ce-9591-5805c0f21ba7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f01993f6-8a28-4c98-99ec-f0183315e229" xlink:to="loc_us-gaap_DebtInstrumentTable_1559d47f-dbc3-45ce-9591-5805c0f21ba7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_884bfb6d-6199-4ef9-897d-9f3ad66f3425" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_1559d47f-dbc3-45ce-9591-5805c0f21ba7" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_884bfb6d-6199-4ef9-897d-9f3ad66f3425" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_884bfb6d-6199-4ef9-897d-9f3ad66f3425_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_884bfb6d-6199-4ef9-897d-9f3ad66f3425" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_884bfb6d-6199-4ef9-897d-9f3ad66f3425_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_855132dd-fdb8-40a8-aac9-2e147df5555e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_884bfb6d-6199-4ef9-897d-9f3ad66f3425" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_855132dd-fdb8-40a8-aac9-2e147df5555e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_583c2a76-5d4b-4ae6-854b-8d0e9f6ed1d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_855132dd-fdb8-40a8-aac9-2e147df5555e" xlink:to="loc_us-gaap_LineOfCreditMember_583c2a76-5d4b-4ae6-854b-8d0e9f6ed1d0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleNotesPayableMember_caa35d31-75a6-43a4-b048-2b334011f098" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleNotesPayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_855132dd-fdb8-40a8-aac9-2e147df5555e" xlink:to="loc_us-gaap_ConvertibleNotesPayableMember_caa35d31-75a6-43a4-b048-2b334011f098" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_d6d96325-d63f-441d-ae88-7ac506a151f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_1559d47f-dbc3-45ce-9591-5805c0f21ba7" xlink:to="loc_us-gaap_DebtInstrumentAxis_d6d96325-d63f-441d-ae88-7ac506a151f5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_d6d96325-d63f-441d-ae88-7ac506a151f5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_d6d96325-d63f-441d-ae88-7ac506a151f5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_d6d96325-d63f-441d-ae88-7ac506a151f5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_a88b7d96-d45d-4681-a422-d6f5e9849f3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_d6d96325-d63f-441d-ae88-7ac506a151f5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_a88b7d96-d45d-4681-a422-d6f5e9849f3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_RevolvingCreditLines2023SeniorSecuredCreditFacilityMember_56313ce9-add4-47eb-bada-6ea1983d9dc8" xlink:href="iiiv-20250630.xsd#iiiv_RevolvingCreditLines2023SeniorSecuredCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_a88b7d96-d45d-4681-a422-d6f5e9849f3c" xlink:to="loc_iiiv_RevolvingCreditLines2023SeniorSecuredCreditFacilityMember_56313ce9-add4-47eb-bada-6ea1983d9dc8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_A1ExchangeableSeniorNotesDue2025Member_4b993fa8-ee7e-48d2-9e01-e890c8259e83" xlink:href="iiiv-20250630.xsd#iiiv_A1ExchangeableSeniorNotesDue2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_a88b7d96-d45d-4681-a422-d6f5e9849f3c" xlink:to="loc_iiiv_A1ExchangeableSeniorNotesDue2025Member_4b993fa8-ee7e-48d2-9e01-e890c8259e83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_7bc5a814-0b59-4e22-87d2-48e84e49dd95" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f01993f6-8a28-4c98-99ec-f0183315e229" xlink:to="loc_us-gaap_LongTermDebt_7bc5a814-0b59-4e22-87d2-48e84e49dd95" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_6ee978aa-9d77-4504-880c-fb69946ca712" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f01993f6-8a28-4c98-99ec-f0183315e229" xlink:to="loc_us-gaap_LongTermDebtCurrent_6ee978aa-9d77-4504-880c-fb69946ca712" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_6d62c831-4c79-4335-8049-ab36438c1519" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f01993f6-8a28-4c98-99ec-f0183315e229" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_6d62c831-4c79-4335-8049-ab36438c1519" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_ae8db470-af28-43a0-8c67-1f0bd549800c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f01993f6-8a28-4c98-99ec-f0183315e229" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_ae8db470-af28-43a0-8c67-1f0bd549800c" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/LONGTERMDEBTNETAdditionalInformationDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#LONGTERMDEBTNETAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/LONGTERMDEBTNETAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_951999ea-4f48-46a7-9d47-153cba2f327c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_DebtInstrumentTable_951999ea-4f48-46a7-9d47-153cba2f327c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_1724e36c-bdc2-40e7-8676-3002e2904837" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_951999ea-4f48-46a7-9d47-153cba2f327c" xlink:to="loc_us-gaap_DebtInstrumentAxis_1724e36c-bdc2-40e7-8676-3002e2904837" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_1724e36c-bdc2-40e7-8676-3002e2904837_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_1724e36c-bdc2-40e7-8676-3002e2904837" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_1724e36c-bdc2-40e7-8676-3002e2904837_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_79fb2efd-e162-4cd1-b06a-b38aa75bb060" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_1724e36c-bdc2-40e7-8676-3002e2904837" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_79fb2efd-e162-4cd1-b06a-b38aa75bb060" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_A1ExchangeableSeniorNotesDue2025Member_f24fbb69-b7b7-4923-b380-c635e4f64e20" xlink:href="iiiv-20250630.xsd#iiiv_A1ExchangeableSeniorNotesDue2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_79fb2efd-e162-4cd1-b06a-b38aa75bb060" xlink:to="loc_iiiv_A1ExchangeableSeniorNotesDue2025Member_f24fbb69-b7b7-4923-b380-c635e4f64e20" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_RevolvingCreditLines2023SeniorSecuredCreditFacilityMember_e478cfbf-7412-42b0-88c1-f3aeab77eafd" xlink:href="iiiv-20250630.xsd#iiiv_RevolvingCreditLines2023SeniorSecuredCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_79fb2efd-e162-4cd1-b06a-b38aa75bb060" xlink:to="loc_iiiv_RevolvingCreditLines2023SeniorSecuredCreditFacilityMember_e478cfbf-7412-42b0-88c1-f3aeab77eafd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_9c7bc118-ade8-4619-b994-9563ff5def62" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_951999ea-4f48-46a7-9d47-153cba2f327c" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_9c7bc118-ade8-4619-b994-9563ff5def62" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9c7bc118-ade8-4619-b994-9563ff5def62_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_9c7bc118-ade8-4619-b994-9563ff5def62" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9c7bc118-ade8-4619-b994-9563ff5def62_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_d827a281-8394-4cf3-bccd-bbbfe4c80546" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_9c7bc118-ade8-4619-b994-9563ff5def62" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_d827a281-8394-4cf3-bccd-bbbfe4c80546" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleNotesPayableMember_b95f2a02-12bf-458b-992c-8c7de448f0ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleNotesPayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_d827a281-8394-4cf3-bccd-bbbfe4c80546" xlink:to="loc_us-gaap_ConvertibleNotesPayableMember_b95f2a02-12bf-458b-992c-8c7de448f0ee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_211f0222-972a-4290-86f3-29c667031ba8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_d827a281-8394-4cf3-bccd-bbbfe4c80546" xlink:to="loc_us-gaap_LineOfCreditMember_211f0222-972a-4290-86f3-29c667031ba8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_5c67dc7e-514d-4171-8173-b4405823c98a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_951999ea-4f48-46a7-9d47-153cba2f327c" xlink:to="loc_us-gaap_StatementClassOfStockAxis_5c67dc7e-514d-4171-8173-b4405823c98a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_5c67dc7e-514d-4171-8173-b4405823c98a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_5c67dc7e-514d-4171-8173-b4405823c98a" xlink:to="loc_us-gaap_ClassOfStockDomain_5c67dc7e-514d-4171-8173-b4405823c98a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_3d49d6c2-ec17-4af7-acba-70ed77a3167f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_5c67dc7e-514d-4171-8173-b4405823c98a" xlink:to="loc_us-gaap_ClassOfStockDomain_3d49d6c2-ec17-4af7-acba-70ed77a3167f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_e00225a5-7ace-472d-9aa9-7a395d4b43e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_3d49d6c2-ec17-4af7-acba-70ed77a3167f" xlink:to="loc_us-gaap_CommonClassAMember_e00225a5-7ace-472d-9aa9-7a395d4b43e9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_ebd1d7f1-e695-476d-9d96-7715bc2f125c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_951999ea-4f48-46a7-9d47-153cba2f327c" xlink:to="loc_us-gaap_CreditFacilityAxis_ebd1d7f1-e695-476d-9d96-7715bc2f125c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_ebd1d7f1-e695-476d-9d96-7715bc2f125c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_ebd1d7f1-e695-476d-9d96-7715bc2f125c" xlink:to="loc_us-gaap_CreditFacilityDomain_ebd1d7f1-e695-476d-9d96-7715bc2f125c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_67883046-ecc5-4b3e-af7c-a2acd7e8f60e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_ebd1d7f1-e695-476d-9d96-7715bc2f125c" xlink:to="loc_us-gaap_CreditFacilityDomain_67883046-ecc5-4b3e-af7c-a2acd7e8f60e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_b3b4d705-23f5-42e0-b0d8-c10f98ad4027" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_67883046-ecc5-4b3e-af7c-a2acd7e8f60e" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_b3b4d705-23f5-42e0-b0d8-c10f98ad4027" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_e5c3ade4-cdde-4ea1-9261-a9fedf2197a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_951999ea-4f48-46a7-9d47-153cba2f327c" xlink:to="loc_us-gaap_VariableRateAxis_e5c3ade4-cdde-4ea1-9261-a9fedf2197a0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_e5c3ade4-cdde-4ea1-9261-a9fedf2197a0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_e5c3ade4-cdde-4ea1-9261-a9fedf2197a0" xlink:to="loc_us-gaap_VariableRateDomain_e5c3ade4-cdde-4ea1-9261-a9fedf2197a0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_3bdd3be4-538d-4979-af89-ffb706f36a64" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_e5c3ade4-cdde-4ea1-9261-a9fedf2197a0" xlink:to="loc_us-gaap_VariableRateDomain_3bdd3be4-538d-4979-af89-ffb706f36a64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_OneTwoThreeOrSixMonthSOFRMember_e6d890d7-5bc2-4770-8a4b-4ffe561521a2" xlink:href="iiiv-20250630.xsd#iiiv_OneTwoThreeOrSixMonthSOFRMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_3bdd3be4-538d-4979-af89-ffb706f36a64" xlink:to="loc_iiiv_OneTwoThreeOrSixMonthSOFRMember_e6d890d7-5bc2-4770-8a4b-4ffe561521a2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AdjustedTermSOFRMember_bc19a338-b7bc-4cf3-8d79-97997c4f1061" xlink:href="iiiv-20250630.xsd#iiiv_AdjustedTermSOFRMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_3bdd3be4-538d-4979-af89-ffb706f36a64" xlink:to="loc_iiiv_AdjustedTermSOFRMember_bc19a338-b7bc-4cf3-8d79-97997c4f1061" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_764661e8-db13-4fb9-9a05-f081c89341f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_3bdd3be4-538d-4979-af89-ffb706f36a64" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_764661e8-db13-4fb9-9a05-f081c89341f8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_SOFRPlus1Member_c9bad344-984c-431c-8fd2-9cbd22f776da" xlink:href="iiiv-20250630.xsd#iiiv_SOFRPlus1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_3bdd3be4-538d-4979-af89-ffb706f36a64" xlink:to="loc_iiiv_SOFRPlus1Member_c9bad344-984c-431c-8fd2-9cbd22f776da" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_36b27d33-342a-418d-bf12-ef2a6a41c64f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_3bdd3be4-538d-4979-af89-ffb706f36a64" xlink:to="loc_us-gaap_BaseRateMember_36b27d33-342a-418d-bf12-ef2a6a41c64f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FederalFundsEffectiveSwapRateMember_a5c84edb-36c0-40d4-a792-10bdfb55717e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalFundsEffectiveSwapRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_3bdd3be4-538d-4979-af89-ffb706f36a64" xlink:to="loc_us-gaap_FederalFundsEffectiveSwapRateMember_a5c84edb-36c0-40d4-a792-10bdfb55717e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_9c79962c-c7be-4957-84ff-fd86d7a29374" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_951999ea-4f48-46a7-9d47-153cba2f327c" xlink:to="loc_srt_RangeAxis_9c79962c-c7be-4957-84ff-fd86d7a29374" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_9c79962c-c7be-4957-84ff-fd86d7a29374_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_9c79962c-c7be-4957-84ff-fd86d7a29374" xlink:to="loc_srt_RangeMember_9c79962c-c7be-4957-84ff-fd86d7a29374_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_663ddaaa-3343-41dc-bdf9-57395a7321ad" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_9c79962c-c7be-4957-84ff-fd86d7a29374" xlink:to="loc_srt_RangeMember_663ddaaa-3343-41dc-bdf9-57395a7321ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_6c4be45e-308d-415b-9776-c0ab6b9add1e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_663ddaaa-3343-41dc-bdf9-57395a7321ad" xlink:to="loc_srt_MinimumMember_6c4be45e-308d-415b-9776-c0ab6b9add1e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_bf3cbdaf-a3f3-45a5-82aa-6e27f66a2250" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_663ddaaa-3343-41dc-bdf9-57395a7321ad" xlink:to="loc_srt_MaximumMember_bf3cbdaf-a3f3-45a5-82aa-6e27f66a2250" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_88c4f587-11f3-4ce6-b9c2-c0a79a05cdc9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_88c4f587-11f3-4ce6-b9c2-c0a79a05cdc9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_ef4d3ba8-c7c1-4f0c-ae45-3c16d9a6a88f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_ef4d3ba8-c7c1-4f0c-ae45-3c16d9a6a88f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromConvertibleDebt_bbb16ad8-2dcf-4d9e-aa7d-199177e9cca0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromConvertibleDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_ProceedsFromConvertibleDebt_bbb16ad8-2dcf-4d9e-aa7d-199177e9cca0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRepurchaseAmount_89b445d5-c03f-43e9-8f54-f20c7478b830" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRepurchaseAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_DebtInstrumentRepurchaseAmount_89b445d5-c03f-43e9-8f54-f20c7478b830" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfConvertibleDebt_99ca10b3-fe53-4874-b168-37d5c2d911de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfConvertibleDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_RepaymentsOfConvertibleDebt_99ca10b3-fe53-4874-b168-37d5c2d911de" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtInstrumentAccruedInterestRepurchaseAmount_5d1e6bb1-20a6-442a-9c8e-9fe3507ab1e8" xlink:href="iiiv-20250630.xsd#iiiv_DebtInstrumentAccruedInterestRepurchaseAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_iiiv_DebtInstrumentAccruedInterestRepurchaseAmount_5d1e6bb1-20a6-442a-9c8e-9fe3507ab1e8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WriteOffOfDeferredDebtIssuanceCost_2a338751-c866-4a8b-80d4-232402d5a901" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WriteOffOfDeferredDebtIssuanceCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_WriteOffOfDeferredDebtIssuanceCost_2a338751-c866-4a8b-80d4-232402d5a901" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_5d3a9f20-8d44-4891-87d5-25e5053efc29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_5d3a9f20-8d44-4891-87d5-25e5053efc29" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_91428234-bad7-4ea1-b1fc-4358cd00b54d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_InterestExpenseDebt_91428234-bad7-4ea1-b1fc-4358cd00b54d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForHedgeFinancingActivities_757e4814-105f-45a7-8d55-529a6d93f6cf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForHedgeFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_PaymentsForHedgeFinancingActivities_757e4814-105f-45a7-8d55-529a6d93f6cf" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromHedgeFinancingActivities_6b3997fd-4bbe-4a0b-8c5d-537ec1638ab4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromHedgeFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_ProceedsFromHedgeFinancingActivities_6b3997fd-4bbe-4a0b-8c5d-537ec1638ab4" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfDerivatives_24fd2bdc-b8e6-4649-bb1e-f0d1b41beea6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSaleOfDerivatives"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_GainLossOnSaleOfDerivatives_24fd2bdc-b8e6-4649-bb1e-f0d1b41beea6" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_0f5e7be0-1e37-4811-ae55-09345dc0095a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_0f5e7be0-1e37-4811-ae55-09345dc0095a" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_1df374e1-65cb-44ed-8b84-cf974f39f5b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_1df374e1-65cb-44ed-8b84-cf974f39f5b4" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_18f8e55e-113e-4ba4-96f0-333bd4560253" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_18f8e55e-113e-4ba4-96f0-333bd4560253" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfWarrants_76d5e83a-d653-451a-ac22-968f11022417" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfWarrants_76d5e83a-d653-451a-ac22-968f11022417" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForRepurchaseOfWarrants_0c7b28d6-1b6e-4713-b333-cc2bdfb4ef43" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForRepurchaseOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_PaymentsForRepurchaseOfWarrants_0c7b28d6-1b6e-4713-b333-cc2bdfb4ef43" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_GainLossOnRepurchaseOfWarrants_b29980e3-96e9-4772-9c9b-c275cb68c218" xlink:href="iiiv-20250630.xsd#iiiv_GainLossOnRepurchaseOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_iiiv_GainLossOnRepurchaseOfWarrants_b29980e3-96e9-4772-9c9b-c275cb68c218" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_8b858f6a-c3ab-4f1c-82de-84b7f2a3b3f1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_8b858f6a-c3ab-4f1c-82de-84b7f2a3b3f1" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_LineOfCreditOptionalAdditionToBorrowingCapacity_bc05da79-c2ca-4737-8e81-9f2e2aa0c57e" xlink:href="iiiv-20250630.xsd#iiiv_LineOfCreditOptionalAdditionToBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_iiiv_LineOfCreditOptionalAdditionToBorrowingCapacity_bc05da79-c2ca-4737-8e81-9f2e2aa0c57e" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_LineOfCreditFacilityConsolidationBasis_f5b8336f-ed27-4ece-b0c8-045f09ca878e" xlink:href="iiiv-20250630.xsd#iiiv_LineOfCreditFacilityConsolidationBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_iiiv_LineOfCreditFacilityConsolidationBasis_f5b8336f-ed27-4ece-b0c8-045f09ca878e" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtCovenantMinimumConsolidatedInterestCoverageRatio_b0dff0be-5b24-4ef8-b963-466b78c06ad9" xlink:href="iiiv-20250630.xsd#iiiv_DebtCovenantMinimumConsolidatedInterestCoverageRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_iiiv_DebtCovenantMinimumConsolidatedInterestCoverageRatio_b0dff0be-5b24-4ef8-b963-466b78c06ad9" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtCovenantMaximumTotalLeverageRatio_0608fdd8-7c5f-4428-b49b-016e841af427" xlink:href="iiiv-20250630.xsd#iiiv_DebtCovenantMaximumTotalLeverageRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_iiiv_DebtCovenantMaximumTotalLeverageRatio_0608fdd8-7c5f-4428-b49b-016e841af427" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_1f5da1f9-32b9-440c-81be-4f2a49359885" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_1f5da1f9-32b9-440c-81be-4f2a49359885" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_0779d359-b2a3-4a69-8004-0abca3e61089" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_0779d359-b2a3-4a69-8004-0abca3e61089" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtInstrumentCovenantCommitmentFeeMultiplier_8e9533da-7984-4dc3-b506-2966c0ce0b6e" xlink:href="iiiv-20250630.xsd#iiiv_DebtInstrumentCovenantCommitmentFeeMultiplier"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_iiiv_DebtInstrumentCovenantCommitmentFeeMultiplier_8e9533da-7984-4dc3-b506-2966c0ce0b6e" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtIssuanceCostsIncurredInPeriod_d01ca486-1e62-4254-a9a2-bc1f45bf4410" xlink:href="iiiv-20250630.xsd#iiiv_DebtIssuanceCostsIncurredInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_iiiv_DebtIssuanceCostsIncurredInPeriod_d01ca486-1e62-4254-a9a2-bc1f45bf4410" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_4a036c98-5b0a-4034-956e-2371f98c2f32" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9e76d5-b136-48b5-b8cf-1fcaff66d751" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_4a036c98-5b0a-4034-956e-2371f98c2f32" xlink:type="arc" order="27"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/LONGTERMDEBTNETConsolidatedTotalNetLeverageRatioDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#LONGTERMDEBTNETConsolidatedTotalNetLeverageRatioDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/LONGTERMDEBTNETConsolidatedTotalNetLeverageRatioDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_0a49ac5e-ec27-4e46-b963-4f3045d1bf69" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_b5172dbe-d046-4f3c-b0b0-2693f2910eae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_0a49ac5e-ec27-4e46-b963-4f3045d1bf69" xlink:to="loc_us-gaap_DebtInstrumentTable_b5172dbe-d046-4f3c-b0b0-2693f2910eae" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtInstrumentLeverageRatioAxis_e59d0b29-e0d0-4170-9ec5-790e906288c8" xlink:href="iiiv-20250630.xsd#iiiv_DebtInstrumentLeverageRatioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_b5172dbe-d046-4f3c-b0b0-2693f2910eae" xlink:to="loc_iiiv_DebtInstrumentLeverageRatioAxis_e59d0b29-e0d0-4170-9ec5-790e906288c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtInstrumentLeverageRatioDomain_e59d0b29-e0d0-4170-9ec5-790e906288c8_default" xlink:href="iiiv-20250630.xsd#iiiv_DebtInstrumentLeverageRatioDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_iiiv_DebtInstrumentLeverageRatioAxis_e59d0b29-e0d0-4170-9ec5-790e906288c8" xlink:to="loc_iiiv_DebtInstrumentLeverageRatioDomain_e59d0b29-e0d0-4170-9ec5-790e906288c8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtInstrumentLeverageRatioDomain_29b2a6db-734f-4d92-b0db-04c20910404b" xlink:href="iiiv-20250630.xsd#iiiv_DebtInstrumentLeverageRatioDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_iiiv_DebtInstrumentLeverageRatioAxis_e59d0b29-e0d0-4170-9ec5-790e906288c8" xlink:to="loc_iiiv_DebtInstrumentLeverageRatioDomain_29b2a6db-734f-4d92-b0db-04c20910404b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtInstrumentLeverageRatioOneMember_b4c1bd26-0875-468f-81d3-4aa346c048a6" xlink:href="iiiv-20250630.xsd#iiiv_DebtInstrumentLeverageRatioOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DebtInstrumentLeverageRatioDomain_29b2a6db-734f-4d92-b0db-04c20910404b" xlink:to="loc_iiiv_DebtInstrumentLeverageRatioOneMember_b4c1bd26-0875-468f-81d3-4aa346c048a6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtInstrumentLeverageRatioTwoMember_147f6158-4089-4d33-87fe-d95b3b14e20e" xlink:href="iiiv-20250630.xsd#iiiv_DebtInstrumentLeverageRatioTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DebtInstrumentLeverageRatioDomain_29b2a6db-734f-4d92-b0db-04c20910404b" xlink:to="loc_iiiv_DebtInstrumentLeverageRatioTwoMember_147f6158-4089-4d33-87fe-d95b3b14e20e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtInstrumentLeverageRatioThreeMember_5222c2a0-91a7-4d2b-923f-d8b7c95c05ed" xlink:href="iiiv-20250630.xsd#iiiv_DebtInstrumentLeverageRatioThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DebtInstrumentLeverageRatioDomain_29b2a6db-734f-4d92-b0db-04c20910404b" xlink:to="loc_iiiv_DebtInstrumentLeverageRatioThreeMember_5222c2a0-91a7-4d2b-923f-d8b7c95c05ed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtInstrumentLeverageRatioFourMember_5017ca01-c4ae-4d64-8c64-239d7ad1e89d" xlink:href="iiiv-20250630.xsd#iiiv_DebtInstrumentLeverageRatioFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_DebtInstrumentLeverageRatioDomain_29b2a6db-734f-4d92-b0db-04c20910404b" xlink:to="loc_iiiv_DebtInstrumentLeverageRatioFourMember_5017ca01-c4ae-4d64-8c64-239d7ad1e89d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_e6a304c7-e36b-4fa6-8532-78b618e98a31" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_b5172dbe-d046-4f3c-b0b0-2693f2910eae" xlink:to="loc_us-gaap_CreditFacilityAxis_e6a304c7-e36b-4fa6-8532-78b618e98a31" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_e6a304c7-e36b-4fa6-8532-78b618e98a31_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_e6a304c7-e36b-4fa6-8532-78b618e98a31" xlink:to="loc_us-gaap_CreditFacilityDomain_e6a304c7-e36b-4fa6-8532-78b618e98a31_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_c291784e-8db7-4131-ac43-39337c6d8ef8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_e6a304c7-e36b-4fa6-8532-78b618e98a31" xlink:to="loc_us-gaap_CreditFacilityDomain_c291784e-8db7-4131-ac43-39337c6d8ef8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_74e9e660-1ca2-4009-baa5-bb9623fcfbde" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_c291784e-8db7-4131-ac43-39337c6d8ef8" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_74e9e660-1ca2-4009-baa5-bb9623fcfbde" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_917489e3-b8ba-49b0-aede-15c4a9f0159c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_b5172dbe-d046-4f3c-b0b0-2693f2910eae" xlink:to="loc_us-gaap_DebtInstrumentAxis_917489e3-b8ba-49b0-aede-15c4a9f0159c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_917489e3-b8ba-49b0-aede-15c4a9f0159c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_917489e3-b8ba-49b0-aede-15c4a9f0159c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_917489e3-b8ba-49b0-aede-15c4a9f0159c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_703ed6fe-8773-4ca3-8a2d-67573fa5e128" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_917489e3-b8ba-49b0-aede-15c4a9f0159c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_703ed6fe-8773-4ca3-8a2d-67573fa5e128" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_RevolvingCreditLines2023SeniorSecuredCreditFacilityMember_c8d22618-d258-4ebc-a1bf-dd535da221f3" xlink:href="iiiv-20250630.xsd#iiiv_RevolvingCreditLines2023SeniorSecuredCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_703ed6fe-8773-4ca3-8a2d-67573fa5e128" xlink:to="loc_iiiv_RevolvingCreditLines2023SeniorSecuredCreditFacilityMember_c8d22618-d258-4ebc-a1bf-dd535da221f3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_58ecf37d-df29-4e66-b16e-41ac09a98b00" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_b5172dbe-d046-4f3c-b0b0-2693f2910eae" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_58ecf37d-df29-4e66-b16e-41ac09a98b00" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_58ecf37d-df29-4e66-b16e-41ac09a98b00_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_58ecf37d-df29-4e66-b16e-41ac09a98b00" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_58ecf37d-df29-4e66-b16e-41ac09a98b00_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_daee1880-6904-4a84-8100-6f3717edec2d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_58ecf37d-df29-4e66-b16e-41ac09a98b00" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_daee1880-6904-4a84-8100-6f3717edec2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_f4787efa-2e59-478b-88c2-a6ff886ac43d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_daee1880-6904-4a84-8100-6f3717edec2d" xlink:to="loc_us-gaap_LineOfCreditMember_f4787efa-2e59-478b-88c2-a6ff886ac43d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_a56b5238-8b56-496e-9fad-9f0059f26a52" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_daee1880-6904-4a84-8100-6f3717edec2d" xlink:to="loc_us-gaap_LetterOfCreditMember_a56b5238-8b56-496e-9fad-9f0059f26a52" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_f815688c-d814-4221-afe4-1692d3fcad50" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_daee1880-6904-4a84-8100-6f3717edec2d" xlink:to="loc_us-gaap_SecuredDebtMember_f815688c-d814-4221-afe4-1692d3fcad50" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_d25245c9-359f-40b9-b78e-6ec03cdf054f" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_b5172dbe-d046-4f3c-b0b0-2693f2910eae" xlink:to="loc_srt_RangeAxis_d25245c9-359f-40b9-b78e-6ec03cdf054f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_d25245c9-359f-40b9-b78e-6ec03cdf054f_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_d25245c9-359f-40b9-b78e-6ec03cdf054f" xlink:to="loc_srt_RangeMember_d25245c9-359f-40b9-b78e-6ec03cdf054f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_b7139aab-fcb5-4335-bb9d-3069d83d80f0" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_d25245c9-359f-40b9-b78e-6ec03cdf054f" xlink:to="loc_srt_RangeMember_b7139aab-fcb5-4335-bb9d-3069d83d80f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_1751f81d-2c0c-43de-8e6f-4e10a626d3e1" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b7139aab-fcb5-4335-bb9d-3069d83d80f0" xlink:to="loc_srt_MinimumMember_1751f81d-2c0c-43de-8e6f-4e10a626d3e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_6be335fe-e5c3-4d6a-af9c-4276a95de30d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b7139aab-fcb5-4335-bb9d-3069d83d80f0" xlink:to="loc_srt_MaximumMember_6be335fe-e5c3-4d6a-af9c-4276a95de30d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DebtInstrumentCovenantNetLeverageRatio_c9f55c9a-98ab-4b70-8f38-f28b6848d375" xlink:href="iiiv-20250630.xsd#iiiv_DebtInstrumentCovenantNetLeverageRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_0a49ac5e-ec27-4e46-b963-4f3045d1bf69" xlink:to="loc_iiiv_DebtInstrumentCovenantNetLeverageRatio_c9f55c9a-98ab-4b70-8f38-f28b6848d375" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_59733ca3-8726-4ce7-b11a-6118344d1e82" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_0a49ac5e-ec27-4e46-b963-4f3045d1bf69" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_59733ca3-8726-4ce7-b11a-6118344d1e82" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_LineOfCreditFacilityLetterOfCreditFeePercentage_63b14a28-2262-487c-b64d-66a62101dd91" xlink:href="iiiv-20250630.xsd#iiiv_LineOfCreditFacilityLetterOfCreditFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_0a49ac5e-ec27-4e46-b963-4f3045d1bf69" xlink:to="loc_iiiv_LineOfCreditFacilityLetterOfCreditFeePercentage_63b14a28-2262-487c-b64d-66a62101dd91" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_1383c8ce-7979-46c8-b8a6-9d13b1afcf5d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_0a49ac5e-ec27-4e46-b963-4f3045d1bf69" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_1383c8ce-7979-46c8-b8a6-9d13b1afcf5d" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/STOCKHOLDERSEQUITYDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#STOCKHOLDERSEQUITYDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/STOCKHOLDERSEQUITYDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramLineItems_b1bd2c86-038b-4bbf-b201-7442273b5657" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramTable_bbe11129-7cc9-4325-81d6-ca638f256b03" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_b1bd2c86-038b-4bbf-b201-7442273b5657" xlink:to="loc_srt_ShareRepurchaseProgramTable_bbe11129-7cc9-4325-81d6-ca638f256b03" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramAxis_14df1f84-941f-47d9-a8ca-d99dd92a7ddd" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ShareRepurchaseProgramTable_bbe11129-7cc9-4325-81d6-ca638f256b03" xlink:to="loc_srt_ShareRepurchaseProgramAxis_14df1f84-941f-47d9-a8ca-d99dd92a7ddd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_14df1f84-941f-47d9-a8ca-d99dd92a7ddd_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ShareRepurchaseProgramAxis_14df1f84-941f-47d9-a8ca-d99dd92a7ddd" xlink:to="loc_srt_ShareRepurchaseProgramDomain_14df1f84-941f-47d9-a8ca-d99dd92a7ddd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_d12ca038-6c82-465d-9826-1c8de1166441" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ShareRepurchaseProgramAxis_14df1f84-941f-47d9-a8ca-d99dd92a7ddd" xlink:to="loc_srt_ShareRepurchaseProgramDomain_d12ca038-6c82-465d-9826-1c8de1166441" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_A2024NewShareRepurchaseProgramMember_28516247-1599-4fcd-b600-d9617c86a5bf" xlink:href="iiiv-20250630.xsd#iiiv_A2024NewShareRepurchaseProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramDomain_d12ca038-6c82-465d-9826-1c8de1166441" xlink:to="loc_iiiv_A2024NewShareRepurchaseProgramMember_28516247-1599-4fcd-b600-d9617c86a5bf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_c17b55ed-e178-4b42-9b5c-411ed48bbcc7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ShareRepurchaseProgramTable_bbe11129-7cc9-4325-81d6-ca638f256b03" xlink:to="loc_us-gaap_StatementClassOfStockAxis_c17b55ed-e178-4b42-9b5c-411ed48bbcc7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_c17b55ed-e178-4b42-9b5c-411ed48bbcc7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_c17b55ed-e178-4b42-9b5c-411ed48bbcc7" xlink:to="loc_us-gaap_ClassOfStockDomain_c17b55ed-e178-4b42-9b5c-411ed48bbcc7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_bdda9c5f-4274-4587-94d2-768c34225ce8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_c17b55ed-e178-4b42-9b5c-411ed48bbcc7" xlink:to="loc_us-gaap_ClassOfStockDomain_bdda9c5f-4274-4587-94d2-768c34225ce8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_3f160240-000a-4afa-bedd-c8fa83598047" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_bdda9c5f-4274-4587-94d2-768c34225ce8" xlink:to="loc_us-gaap_CommonClassAMember_3f160240-000a-4afa-bedd-c8fa83598047" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StockRepurchaseProgramAuthorizedAmount1_d62f4ae9-f2eb-4f22-93fa-8643a4e9b947" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_b1bd2c86-038b-4bbf-b201-7442273b5657" xlink:to="loc_srt_StockRepurchaseProgramAuthorizedAmount1_d62f4ae9-f2eb-4f22-93fa-8643a4e9b947" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_0a02f735-e490-48b3-ba4b-39d8b9699fe4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockSharesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_b1bd2c86-038b-4bbf-b201-7442273b5657" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_0a02f735-e490-48b3-ba4b-39d8b9699fe4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_57f0c423-cf03-49af-a55d-edbc74c43979" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_b1bd2c86-038b-4bbf-b201-7442273b5657" xlink:to="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_57f0c423-cf03-49af-a55d-edbc74c43979" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_5fbcdd40-6136-40bf-97e4-ed39f75dc54c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_b1bd2c86-038b-4bbf-b201-7442273b5657" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_5fbcdd40-6136-40bf-97e4-ed39f75dc54c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_d28fae42-3821-4af5-b7e4-b008cd494e62" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_b1bd2c86-038b-4bbf-b201-7442273b5657" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_d28fae42-3821-4af5-b7e4-b008cd494e62" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/INCOMETAXESDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#INCOMETAXESDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/INCOMETAXESDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_fe140544-be3e-49de-9a88-e451aa9bf3ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_fe140544-be3e-49de-9a88-e451aa9bf3ae" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_cc60c7be-367c-4a6d-8df6-6a5de4f06817" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_fe140544-be3e-49de-9a88-e451aa9bf3ae" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_cc60c7be-367c-4a6d-8df6-6a5de4f06817" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_cc60c7be-367c-4a6d-8df6-6a5de4f06817_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_cc60c7be-367c-4a6d-8df6-6a5de4f06817" xlink:to="loc_us-gaap_EquityComponentDomain_cc60c7be-367c-4a6d-8df6-6a5de4f06817_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_f267605f-bfa2-4340-9c86-e8d454809ea3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_cc60c7be-367c-4a6d-8df6-6a5de4f06817" xlink:to="loc_us-gaap_EquityComponentDomain_f267605f-bfa2-4340-9c86-e8d454809ea3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_c3122da8-7436-4bde-942f-ef44779b1d00" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_f267605f-bfa2-4340-9c86-e8d454809ea3" xlink:to="loc_us-gaap_CommonStockMember_c3122da8-7436-4bde-942f-ef44779b1d00" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_77276a0c-9f5a-4abc-966a-af4cb3bf0fa5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_fe140544-be3e-49de-9a88-e451aa9bf3ae" xlink:to="loc_us-gaap_StatementClassOfStockAxis_77276a0c-9f5a-4abc-966a-af4cb3bf0fa5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_77276a0c-9f5a-4abc-966a-af4cb3bf0fa5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_77276a0c-9f5a-4abc-966a-af4cb3bf0fa5" xlink:to="loc_us-gaap_ClassOfStockDomain_77276a0c-9f5a-4abc-966a-af4cb3bf0fa5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_33719f0c-827f-4ca3-b8df-ecaf72393800" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_77276a0c-9f5a-4abc-966a-af4cb3bf0fa5" xlink:to="loc_us-gaap_ClassOfStockDomain_33719f0c-827f-4ca3-b8df-ecaf72393800" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_c4fbfebe-5b6d-4091-a32a-5347156fb469" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_33719f0c-827f-4ca3-b8df-ecaf72393800" xlink:to="loc_us-gaap_CommonClassAMember_c4fbfebe-5b6d-4091-a32a-5347156fb469" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_84ba1839-7d00-4d5f-bbba-29aee1169cc8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_33719f0c-827f-4ca3-b8df-ecaf72393800" xlink:to="loc_us-gaap_CommonClassBMember_84ba1839-7d00-4d5f-bbba-29aee1169cc8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_5243ba8b-9df6-4c90-8493-d11562cd8851" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_fe140544-be3e-49de-9a88-e451aa9bf3ae" xlink:to="loc_srt_CounterpartyNameAxis_5243ba8b-9df6-4c90-8493-d11562cd8851" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_5243ba8b-9df6-4c90-8493-d11562cd8851_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_5243ba8b-9df6-4c90-8493-d11562cd8851" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_5243ba8b-9df6-4c90-8493-d11562cd8851_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_bebfa37f-76e7-4529-ab80-dc2e381dafbc" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_5243ba8b-9df6-4c90-8493-d11562cd8851" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_bebfa37f-76e7-4529-ab80-dc2e381dafbc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_ContinuingEquityOwnersMember_4bff78a9-10d1-4407-bc11-74a62101c7e0" xlink:href="iiiv-20250630.xsd#iiiv_ContinuingEquityOwnersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_bebfa37f-76e7-4529-ab80-dc2e381dafbc" xlink:to="loc_iiiv_ContinuingEquityOwnersMember_4bff78a9-10d1-4407-bc11-74a62101c7e0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_5540439f-3d7f-482d-abe0-2e54dd3c304a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_fe140544-be3e-49de-9a88-e451aa9bf3ae" xlink:to="loc_srt_RangeAxis_5540439f-3d7f-482d-abe0-2e54dd3c304a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_5540439f-3d7f-482d-abe0-2e54dd3c304a_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_5540439f-3d7f-482d-abe0-2e54dd3c304a" xlink:to="loc_srt_RangeMember_5540439f-3d7f-482d-abe0-2e54dd3c304a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_15040e8d-eb82-4b5b-8c67-a5fa84aa3cbd" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_5540439f-3d7f-482d-abe0-2e54dd3c304a" xlink:to="loc_srt_RangeMember_15040e8d-eb82-4b5b-8c67-a5fa84aa3cbd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_09ff5fc2-6393-4572-af64-aa4132016c4c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_15040e8d-eb82-4b5b-8c67-a5fa84aa3cbd" xlink:to="loc_srt_MinimumMember_09ff5fc2-6393-4572-af64-aa4132016c4c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_c6815fd1-f6fd-49f2-96e5-bf6459f7cd4d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_15040e8d-eb82-4b5b-8c67-a5fa84aa3cbd" xlink:to="loc_srt_MaximumMember_c6815fd1-f6fd-49f2-96e5-bf6459f7cd4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_8755f94c-f94d-4842-a9e5-01818251c454" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_8755f94c-f94d-4842-a9e5-01818251c454" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_TaxReceivableAgreementPercentOfTaxBenefitsPayableToContinuingEquityOwners_13497c7a-8c5b-41d6-af3d-b0d020fb69c9" xlink:href="iiiv-20250630.xsd#iiiv_TaxReceivableAgreementPercentOfTaxBenefitsPayableToContinuingEquityOwners"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:to="loc_iiiv_TaxReceivableAgreementPercentOfTaxBenefitsPayableToContinuingEquityOwners_13497c7a-8c5b-41d6-af3d-b0d020fb69c9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_TaxReceivableAgreementPercentOfTaxBenefitsRetained_2d711431-0c78-49d1-b70d-5cd52bfbf090" xlink:href="iiiv-20250630.xsd#iiiv_TaxReceivableAgreementPercentOfTaxBenefitsRetained"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:to="loc_iiiv_TaxReceivableAgreementPercentOfTaxBenefitsRetained_2d711431-0c78-49d1-b70d-5cd52bfbf090" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRedeemedOrCalledDuringPeriodShares_06a0b8da-166b-4e5e-9f39-3a283ea1cac3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRedeemedOrCalledDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:to="loc_us-gaap_StockRedeemedOrCalledDuringPeriodShares_06a0b8da-166b-4e5e-9f39-3a283ea1cac3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_IncreaseDecreaseInDeferredTaxAssetsNet_4bd7cd3a-7034-4900-970c-583c1db51a7a" xlink:href="iiiv-20250630.xsd#iiiv_IncreaseDecreaseInDeferredTaxAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:to="loc_iiiv_IncreaseDecreaseInDeferredTaxAssetsNet_4bd7cd3a-7034-4900-970c-583c1db51a7a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_IncreaseDecreaseInTaxBenefitDueToRelatedParties_74a628b0-972a-4836-8655-6f819a0f6183" xlink:href="iiiv-20250630.xsd#iiiv_IncreaseDecreaseInTaxBenefitDueToRelatedParties"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:to="loc_iiiv_IncreaseDecreaseInTaxBenefitDueToRelatedParties_74a628b0-972a-4836-8655-6f819a0f6183" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDistributionsToAffiliates_d0cc2dbc-7757-4655-849b-501575c27f9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDistributionsToAffiliates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:to="loc_us-gaap_PaymentsOfDistributionsToAffiliates_d0cc2dbc-7757-4655-849b-501575c27f9d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsGross_d854ca9f-ef5f-4e73-b478-3ba908df21e7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:to="loc_us-gaap_DeferredTaxAssetsGross_d854ca9f-ef5f-4e73-b478-3ba908df21e7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_TaxBenefitDueToRelatedParties_a06b8cb8-748c-4021-9539-6355ea206318" xlink:href="iiiv-20250630.xsd#iiiv_TaxBenefitDueToRelatedParties"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:to="loc_iiiv_TaxBenefitDueToRelatedParties_a06b8cb8-748c-4021-9539-6355ea206318" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_ExpectedPaymentsForRepurchaseOfRedeemableNoncontrollingInterest_2592f93d-1452-4e42-b1de-ccbd5a2f4fc5" xlink:href="iiiv-20250630.xsd#iiiv_ExpectedPaymentsForRepurchaseOfRedeemableNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:to="loc_iiiv_ExpectedPaymentsForRepurchaseOfRedeemableNoncontrollingInterest_2592f93d-1452-4e42-b1de-ccbd5a2f4fc5" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_PeriodOfPaymentToContinuingEquityOwners_430315e4-6f23-49fd-9782-2e627f168fb8" xlink:href="iiiv-20250630.xsd#iiiv_PeriodOfPaymentToContinuingEquityOwners"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f42f2c9c-20be-4bcf-8758-020444ea2347" xlink:to="loc_iiiv_PeriodOfPaymentToContinuingEquityOwners_430315e4-6f23-49fd-9782-2e627f168fb8" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/FAIRVALUEMEASUREMENTSChangesinLevel3FinancialInstrumentsMeasuredonRecurringBasisDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#FAIRVALUEMEASUREMENTSChangesinLevel3FinancialInstrumentsMeasuredonRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/FAIRVALUEMEASUREMENTSChangesinLevel3FinancialInstrumentsMeasuredonRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_ef11a280-2a57-4a36-880d-3f9678504e9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_8caaf9bf-d0fa-49fc-8d38-d5d71823a50e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_ef11a280-2a57-4a36-880d-3f9678504e9d" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_8caaf9bf-d0fa-49fc-8d38-d5d71823a50e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_f567d821-7796-48ed-bea6-403158db1f25" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_8caaf9bf-d0fa-49fc-8d38-d5d71823a50e" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_f567d821-7796-48ed-bea6-403158db1f25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_f567d821-7796-48ed-bea6-403158db1f25_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_f567d821-7796-48ed-bea6-403158db1f25" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_f567d821-7796-48ed-bea6-403158db1f25_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_96ffbc8c-50a6-4666-9900-60d37038762c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_f567d821-7796-48ed-bea6-403158db1f25" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_96ffbc8c-50a6-4666-9900-60d37038762c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AccruedContingentConsiderationMember_f4a0b6d6-07b3-4bf1-b837-437658082553" xlink:href="iiiv-20250630.xsd#iiiv_AccruedContingentConsiderationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_96ffbc8c-50a6-4666-9900-60d37038762c" xlink:to="loc_iiiv_AccruedContingentConsiderationMember_f4a0b6d6-07b3-4bf1-b837-437658082553" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfFinancialPositionLocationActivityCapitalizationAxis_9da38973-77b9-46f3-9725-0a99e6dd8cb3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementOfFinancialPositionLocationActivityCapitalizationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_8caaf9bf-d0fa-49fc-8d38-d5d71823a50e" xlink:to="loc_us-gaap_StatementOfFinancialPositionLocationActivityCapitalizationAxis_9da38973-77b9-46f3-9725-0a99e6dd8cb3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfFinancialPositionLocationActivityCapitalizationDomain_9da38973-77b9-46f3-9725-0a99e6dd8cb3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementOfFinancialPositionLocationActivityCapitalizationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementOfFinancialPositionLocationActivityCapitalizationAxis_9da38973-77b9-46f3-9725-0a99e6dd8cb3" xlink:to="loc_us-gaap_StatementOfFinancialPositionLocationActivityCapitalizationDomain_9da38973-77b9-46f3-9725-0a99e6dd8cb3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfFinancialPositionLocationActivityCapitalizationDomain_11581a71-d6cb-425c-8e11-8465e0a8165c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementOfFinancialPositionLocationActivityCapitalizationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementOfFinancialPositionLocationActivityCapitalizationAxis_9da38973-77b9-46f3-9725-0a99e6dd8cb3" xlink:to="loc_us-gaap_StatementOfFinancialPositionLocationActivityCapitalizationDomain_11581a71-d6cb-425c-8e11-8465e0a8165c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AccruedContingentConsiderationMember_6828341b-0a64-4764-8f8b-699c84665e10" xlink:href="iiiv-20250630.xsd#iiiv_AccruedContingentConsiderationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementOfFinancialPositionLocationActivityCapitalizationDomain_11581a71-d6cb-425c-8e11-8465e0a8165c" xlink:to="loc_iiiv_AccruedContingentConsiderationMember_6828341b-0a64-4764-8f8b-699c84665e10" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_e4ebd667-12ed-478d-a98f-812619843594" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_8caaf9bf-d0fa-49fc-8d38-d5d71823a50e" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_e4ebd667-12ed-478d-a98f-812619843594" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e4ebd667-12ed-478d-a98f-812619843594_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_e4ebd667-12ed-478d-a98f-812619843594" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_e4ebd667-12ed-478d-a98f-812619843594_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_31396875-026e-4098-ad48-65294abfc8ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_e4ebd667-12ed-478d-a98f-812619843594" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_31396875-026e-4098-ad48-65294abfc8ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_HealthcareRCMBusinessMember_3424ba70-0eee-4acb-bd89-1f4efd72e05c" xlink:href="iiiv-20250630.xsd#iiiv_HealthcareRCMBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_31396875-026e-4098-ad48-65294abfc8ee" xlink:to="loc_iiiv_HealthcareRCMBusinessMember_3424ba70-0eee-4acb-bd89-1f4efd72e05c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_5f7b004a-c621-4097-9afb-3207ed2e3ecb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_ef11a280-2a57-4a36-880d-3f9678504e9d" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_5f7b004a-c621-4097-9afb-3207ed2e3ecb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_f3946873-06e4-49e1-bacd-777523e92e89" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_5f7b004a-c621-4097-9afb-3207ed2e3ecb" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_f3946873-06e4-49e1-bacd-777523e92e89" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_8836117f-56ff-4d7a-8327-26bd1765339f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_5f7b004a-c621-4097-9afb-3207ed2e3ecb" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_8836117f-56ff-4d7a-8327-26bd1765339f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_c5353f1b-edd0-4966-b7fe-d63cf264beda" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_5f7b004a-c621-4097-9afb-3207ed2e3ecb" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_c5353f1b-edd0-4966-b7fe-d63cf264beda" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueRecurringBasisUnobservableInputReconciliationLiabilityGainLossStatementOfIncomeExtensibleList_7e324036-166c-4a34-9d04-84a84e7a4f75" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueRecurringBasisUnobservableInputReconciliationLiabilityGainLossStatementOfIncomeExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_c5353f1b-edd0-4966-b7fe-d63cf264beda" xlink:to="loc_us-gaap_FairValueRecurringBasisUnobservableInputReconciliationLiabilityGainLossStatementOfIncomeExtensibleList_7e324036-166c-4a34-9d04-84a84e7a4f75" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_ce84e83d-d807-45a6-9e32-ebd7ced080b8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_5f7b004a-c621-4097-9afb-3207ed2e3ecb" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_ce84e83d-d807-45a6-9e32-ebd7ced080b8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_b05a2b44-e8b4-449f-b791-608bc2030e5b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationAccruedContingentConsideration_17f46466-dca3-4d80-a218-497185189a67" xlink:href="iiiv-20250630.xsd#iiiv_DisposalGroupIncludingDiscontinuedOperationAccruedContingentConsideration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_ef11a280-2a57-4a36-880d-3f9678504e9d" xlink:to="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationAccruedContingentConsideration_17f46466-dca3-4d80-a218-497185189a67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationAccruedContingentConsiderationCurrentAssetsHeldForSale_9fb918cb-d4cb-41d1-8258-208b854fafd4" xlink:href="iiiv-20250630.xsd#iiiv_DisposalGroupIncludingDiscontinuedOperationAccruedContingentConsiderationCurrentAssetsHeldForSale"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_ef11a280-2a57-4a36-880d-3f9678504e9d" xlink:to="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationAccruedContingentConsiderationCurrentAssetsHeldForSale_9fb918cb-d4cb-41d1-8258-208b854fafd4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationAccruedContingentConsiderationLongTermAssetsHeldForSale_9f58f90c-f24d-4c7d-b469-9a26c1c968cc" xlink:href="iiiv-20250630.xsd#iiiv_DisposalGroupIncludingDiscontinuedOperationAccruedContingentConsiderationLongTermAssetsHeldForSale"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_ef11a280-2a57-4a36-880d-3f9678504e9d" xlink:to="loc_iiiv_DisposalGroupIncludingDiscontinuedOperationAccruedContingentConsiderationLongTermAssetsHeldForSale_9f58f90c-f24d-4c7d-b469-9a26c1c968cc" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/FAIRVALUEMEASUREMENTSAdditionalInformationDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#FAIRVALUEMEASUREMENTSAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/FAIRVALUEMEASUREMENTSAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_edca19cd-ff68-407c-aa16-23c64b45c92e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_3405b6b0-5dfa-459e-9032-3b4dfdede393" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_edca19cd-ff68-407c-aa16-23c64b45c92e" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_3405b6b0-5dfa-459e-9032-3b4dfdede393" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_699f618a-c48b-4002-9644-923c06b2ca12" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_3405b6b0-5dfa-459e-9032-3b4dfdede393" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_699f618a-c48b-4002-9644-923c06b2ca12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_699f618a-c48b-4002-9644-923c06b2ca12_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_699f618a-c48b-4002-9644-923c06b2ca12" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_699f618a-c48b-4002-9644-923c06b2ca12_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_27b6e327-0637-4a5e-b716-4747d8c60d4e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_699f618a-c48b-4002-9644-923c06b2ca12" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_27b6e327-0637-4a5e-b716-4747d8c60d4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_AccruedExpensesAndOtherCurrentLiabilitiesMember_1371cc1f-a9eb-49dd-9b2e-1aef89fa0511" xlink:href="iiiv-20250630.xsd#iiiv_AccruedExpensesAndOtherCurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_27b6e327-0637-4a5e-b716-4747d8c60d4e" xlink:to="loc_iiiv_AccruedExpensesAndOtherCurrentLiabilitiesMember_1371cc1f-a9eb-49dd-9b2e-1aef89fa0511" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_d24439f5-a2ea-4464-9acf-8d26981cf614" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNoncurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_27b6e327-0637-4a5e-b716-4747d8c60d4e" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_d24439f5-a2ea-4464-9acf-8d26981cf614" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_4a0fc608-f61c-4a6d-a70d-f802afeedcc8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_edca19cd-ff68-407c-aa16-23c64b45c92e" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_4a0fc608-f61c-4a6d-a70d-f802afeedcc8" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/EQUITYBASEDCOMPENSATIONSharebasedCompensationExpenseDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#EQUITYBASEDCOMPENSATIONSharebasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/EQUITYBASEDCOMPENSATIONSharebasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_f6acb82d-3d46-4f99-b195-4efc14e4d3a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_0f27109f-42eb-40e5-9627-95aaa35c876a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_f6acb82d-3d46-4f99-b195-4efc14e4d3a6" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_0f27109f-42eb-40e5-9627-95aaa35c876a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_fe1aedd6-e544-4340-84f4-b72a4a1173f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_0f27109f-42eb-40e5-9627-95aaa35c876a" xlink:to="loc_us-gaap_AwardTypeAxis_fe1aedd6-e544-4340-84f4-b72a4a1173f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_fe1aedd6-e544-4340-84f4-b72a4a1173f3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_fe1aedd6-e544-4340-84f4-b72a4a1173f3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_fe1aedd6-e544-4340-84f4-b72a4a1173f3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_459b4b12-e8a0-46ff-8399-d3b9734cfeac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_fe1aedd6-e544-4340-84f4-b72a4a1173f3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_459b4b12-e8a0-46ff-8399-d3b9734cfeac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_3ad3bbc9-f202-479a-8258-e4f8a9302722" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_459b4b12-e8a0-46ff-8399-d3b9734cfeac" xlink:to="loc_us-gaap_EmployeeStockOptionMember_3ad3bbc9-f202-479a-8258-e4f8a9302722" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_042e5c9f-7a6d-4f21-9ac3-a7d2ad2f4902" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_459b4b12-e8a0-46ff-8399-d3b9734cfeac" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_042e5c9f-7a6d-4f21-9ac3-a7d2ad2f4902" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_4352ee60-98a3-4d73-b1cb-25f626d59e9e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_f6acb82d-3d46-4f99-b195-4efc14e4d3a6" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_4352ee60-98a3-4d73-b1cb-25f626d59e9e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/EQUITYBASEDCOMPENSATIONAdditionalInformationDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#EQUITYBASEDCOMPENSATIONAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/EQUITYBASEDCOMPENSATIONAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d3176ded-f193-4af2-81c5-90457f163c3d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d3176ded-f193-4af2-81c5-90457f163c3d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_e4cfe51d-1c32-45f1-a9f6-2e29b8bcb08d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d3176ded-f193-4af2-81c5-90457f163c3d" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_e4cfe51d-1c32-45f1-a9f6-2e29b8bcb08d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_e4cfe51d-1c32-45f1-a9f6-2e29b8bcb08d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_e4cfe51d-1c32-45f1-a9f6-2e29b8bcb08d" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_e4cfe51d-1c32-45f1-a9f6-2e29b8bcb08d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_3da48005-3bc5-4f22-b8bc-a7c552ad7ed3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_e4cfe51d-1c32-45f1-a9f6-2e29b8bcb08d" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_3da48005-3bc5-4f22-b8bc-a7c552ad7ed3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember_b0b5a56d-dd3d-4081-8825-18425467b387" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_3da48005-3bc5-4f22-b8bc-a7c552ad7ed3" xlink:to="loc_us-gaap_DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember_b0b5a56d-dd3d-4081-8825-18425467b387" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_d9480969-a017-466f-aaf8-b4f63bb4d2b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d3176ded-f193-4af2-81c5-90457f163c3d" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_d9480969-a017-466f-aaf8-b4f63bb4d2b7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_d9480969-a017-466f-aaf8-b4f63bb4d2b7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_d9480969-a017-466f-aaf8-b4f63bb4d2b7" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_d9480969-a017-466f-aaf8-b4f63bb4d2b7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_878be45a-aaf5-45a8-8006-a1a153a2ca03" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_d9480969-a017-466f-aaf8-b4f63bb4d2b7" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_878be45a-aaf5-45a8-8006-a1a153a2ca03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_MerchantServicesBusinessMember_d7cb52fb-6be3-44e6-b9d6-497d046238e3" xlink:href="iiiv-20250630.xsd#iiiv_MerchantServicesBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_878be45a-aaf5-45a8-8006-a1a153a2ca03" xlink:to="loc_iiiv_MerchantServicesBusinessMember_d7cb52fb-6be3-44e6-b9d6-497d046238e3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_HealthcareRCMBusinessMember_1b4249ed-1203-43cd-ad7f-b4ee4c4f3409" xlink:href="iiiv-20250630.xsd#iiiv_HealthcareRCMBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_878be45a-aaf5-45a8-8006-a1a153a2ca03" xlink:to="loc_iiiv_HealthcareRCMBusinessMember_1b4249ed-1203-43cd-ad7f-b4ee4c4f3409" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_481a45dd-0300-4c70-ba0b-f0e44a76935e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d3176ded-f193-4af2-81c5-90457f163c3d" xlink:to="loc_us-gaap_AwardTypeAxis_481a45dd-0300-4c70-ba0b-f0e44a76935e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_481a45dd-0300-4c70-ba0b-f0e44a76935e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_481a45dd-0300-4c70-ba0b-f0e44a76935e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_481a45dd-0300-4c70-ba0b-f0e44a76935e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e3c836b0-0023-4e37-907b-da3c6319968e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_481a45dd-0300-4c70-ba0b-f0e44a76935e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e3c836b0-0023-4e37-907b-da3c6319968e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_f49cc483-4e05-443f-a5d3-9f24c3d86552" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e3c836b0-0023-4e37-907b-da3c6319968e" xlink:to="loc_us-gaap_EmployeeStockOptionMember_f49cc483-4e05-443f-a5d3-9f24c3d86552" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_5ab8eac4-8386-4024-8166-c785edfd246f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e3c836b0-0023-4e37-907b-da3c6319968e" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_5ab8eac4-8386-4024-8166-c785edfd246f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_84ce4b37-13a2-47f9-8614-69d3133d1186" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d3176ded-f193-4af2-81c5-90457f163c3d" xlink:to="loc_us-gaap_PlanNameAxis_84ce4b37-13a2-47f9-8614-69d3133d1186" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_84ce4b37-13a2-47f9-8614-69d3133d1186_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_84ce4b37-13a2-47f9-8614-69d3133d1186" xlink:to="loc_us-gaap_PlanNameDomain_84ce4b37-13a2-47f9-8614-69d3133d1186_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_a3e1fb4d-c1ff-4ee4-92a6-8bec98a39583" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_84ce4b37-13a2-47f9-8614-69d3133d1186" xlink:to="loc_us-gaap_PlanNameDomain_a3e1fb4d-c1ff-4ee4-92a6-8bec98a39583" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_A2018EquityIncentiveAwardPlanMember_e4596e27-97a0-4d77-9ac0-2fe36e3e591e" xlink:href="iiiv-20250630.xsd#iiiv_A2018EquityIncentiveAwardPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_a3e1fb4d-c1ff-4ee4-92a6-8bec98a39583" xlink:to="loc_iiiv_A2018EquityIncentiveAwardPlanMember_e4596e27-97a0-4d77-9ac0-2fe36e3e591e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_A2020EquityIncentiveAwardPlanMember_6014c44c-188b-465a-a4de-fae0e5b8c3f9" xlink:href="iiiv-20250630.xsd#iiiv_A2020EquityIncentiveAwardPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_a3e1fb4d-c1ff-4ee4-92a6-8bec98a39583" xlink:to="loc_iiiv_A2020EquityIncentiveAwardPlanMember_6014c44c-188b-465a-a4de-fae0e5b8c3f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_fe807e69-93c8-4684-9f1a-0e794e2f6516" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_fe807e69-93c8-4684-9f1a-0e794e2f6516" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_a0d8604c-40fe-432b-817c-3f833e5d3b17" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_a0d8604c-40fe-432b-817c-3f833e5d3b17" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_1bccb487-0084-499a-b0d3-3b0bda3042f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_1bccb487-0084-499a-b0d3-3b0bda3042f8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantAnnualIncreasePercent_d5bfcd64-71e3-4a93-9af6-69cf3f3a7df4" xlink:href="iiiv-20250630.xsd#iiiv_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantAnnualIncreasePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:to="loc_iiiv_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantAnnualIncreasePercent_d5bfcd64-71e3-4a93-9af6-69cf3f3a7df4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_a02d3d15-db51-449d-ba77-82220f361d8c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_a02d3d15-db51-449d-ba77-82220f361d8c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_eae7d5ea-f367-4339-9c11-de60f21a501f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_eae7d5ea-f367-4339-9c11-de60f21a501f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_7db0f940-78f5-41b6-9b3b-6257d0000fd5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_7db0f940-78f5-41b6-9b3b-6257d0000fd5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_84f5fa35-4a76-4d78-8072-70ddaa50d59f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_84f5fa35-4a76-4d78-8072-70ddaa50d59f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_9ac5a7a6-80ab-4fbf-b935-0bf19455a0cd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_9ac5a7a6-80ab-4fbf-b935-0bf19455a0cd" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_d33f5e05-d580-4b8a-9c9a-bf8ddd0db836" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_d33f5e05-d580-4b8a-9c9a-bf8ddd0db836" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAcceleratedVestingNumber_31b6440f-7077-4927-8792-13624bc572c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAcceleratedVestingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7a6b0796-c9cf-4178-87fc-3a7cbac4fb58" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAcceleratedVestingNumber_31b6440f-7077-4927-8792-13624bc572c4" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/EQUITYBASEDCOMPENSATIONRSUActivityDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#EQUITYBASEDCOMPENSATIONRSUActivityDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/EQUITYBASEDCOMPENSATIONRSUActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9fa1d68e-f348-4fce-9bee-e8e3e185a3f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4a226f40-9e50-43bf-a4ab-b1818ba4386e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9fa1d68e-f348-4fce-9bee-e8e3e185a3f4" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4a226f40-9e50-43bf-a4ab-b1818ba4386e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_b63f0f90-53cd-4153-a1ee-c999b40eb3a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4a226f40-9e50-43bf-a4ab-b1818ba4386e" xlink:to="loc_us-gaap_AwardTypeAxis_b63f0f90-53cd-4153-a1ee-c999b40eb3a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b63f0f90-53cd-4153-a1ee-c999b40eb3a1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_b63f0f90-53cd-4153-a1ee-c999b40eb3a1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b63f0f90-53cd-4153-a1ee-c999b40eb3a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_88e7e046-051f-4134-90f8-82642fdbace2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_b63f0f90-53cd-4153-a1ee-c999b40eb3a1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_88e7e046-051f-4134-90f8-82642fdbace2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_4196ca9e-05d4-4410-9c94-d347c514efcc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_88e7e046-051f-4134-90f8-82642fdbace2" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_4196ca9e-05d4-4410-9c94-d347c514efcc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0cccaf14-06f6-4785-9e27-c40c850e2ce2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9fa1d68e-f348-4fce-9bee-e8e3e185a3f4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0cccaf14-06f6-4785-9e27-c40c850e2ce2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_a1c264e0-120b-456c-813e-856eb0952721" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0cccaf14-06f6-4785-9e27-c40c850e2ce2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_a1c264e0-120b-456c-813e-856eb0952721" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_5a36ae6e-c54b-4156-91a2-185d847e2ac2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0cccaf14-06f6-4785-9e27-c40c850e2ce2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_5a36ae6e-c54b-4156-91a2-185d847e2ac2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_11ad24c9-c83c-4102-b9bc-ee83493ab766" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0cccaf14-06f6-4785-9e27-c40c850e2ce2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_11ad24c9-c83c-4102-b9bc-ee83493ab766" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_27110fd2-bb3c-4ecd-abb3-03678a5424cb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0cccaf14-06f6-4785-9e27-c40c850e2ce2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_27110fd2-bb3c-4ecd-abb3-03678a5424cb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_b2d586eb-e4a2-47fc-be38-d50944569ead" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_fda23377-8ce0-40f1-b247-e783efd21903" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9fa1d68e-f348-4fce-9bee-e8e3e185a3f4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_fda23377-8ce0-40f1-b247-e783efd21903" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_de6b21c2-6869-4c91-8d99-40037a1acad8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_fda23377-8ce0-40f1-b247-e783efd21903" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_de6b21c2-6869-4c91-8d99-40037a1acad8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_12139aed-c9f7-4e53-b924-cf17ae9baaec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_fda23377-8ce0-40f1-b247-e783efd21903" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_12139aed-c9f7-4e53-b924-cf17ae9baaec" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_500c5f00-e61e-419d-a670-f79ee56bd4d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_fda23377-8ce0-40f1-b247-e783efd21903" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_500c5f00-e61e-419d-a670-f79ee56bd4d1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_e4252c9a-b1ec-4eaf-af96-feba20ca793d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_fda23377-8ce0-40f1-b247-e783efd21903" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_e4252c9a-b1ec-4eaf-af96-feba20ca793d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_cde4ab79-bffa-4d23-bf96-1ea65d4f4b6b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/COMMITMENTSANDCONTINGENCIESSSLitigationDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#COMMITMENTSANDCONTINGENCIESSSLitigationDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/COMMITMENTSANDCONTINGENCIESSSLitigationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_ea30a174-d00a-4a21-8d42-67c83052bd44" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_2e962f26-8b5d-479a-883e-c51857f2018f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_ea30a174-d00a-4a21-8d42-67c83052bd44" xlink:to="loc_us-gaap_LossContingenciesTable_2e962f26-8b5d-479a-883e-c51857f2018f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_e8c02c1e-3aaa-4109-bfec-d11dc24a3888" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_2e962f26-8b5d-479a-883e-c51857f2018f" xlink:to="loc_srt_LitigationCaseAxis_e8c02c1e-3aaa-4109-bfec-d11dc24a3888" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_e8c02c1e-3aaa-4109-bfec-d11dc24a3888_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_e8c02c1e-3aaa-4109-bfec-d11dc24a3888" xlink:to="loc_srt_LitigationCaseTypeDomain_e8c02c1e-3aaa-4109-bfec-d11dc24a3888_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_41f316a9-8be0-4a39-a7c3-373281aafe2e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_e8c02c1e-3aaa-4109-bfec-d11dc24a3888" xlink:to="loc_srt_LitigationCaseTypeDomain_41f316a9-8be0-4a39-a7c3-373281aafe2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_SSVsStateMember_5be1f314-9831-4ed9-ac50-6b97ed28604d" xlink:href="iiiv-20250630.xsd#iiiv_SSVsStateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_41f316a9-8be0-4a39-a7c3-373281aafe2e" xlink:to="loc_iiiv_SSVsStateMember_5be1f314-9831-4ed9-ac50-6b97ed28604d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_SSVsSheriffsMember_1a8d62b6-e673-4dc5-9a27-a675032f7f61" xlink:href="iiiv-20250630.xsd#iiiv_SSVsSheriffsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_41f316a9-8be0-4a39-a7c3-373281aafe2e" xlink:to="loc_iiiv_SSVsSheriffsMember_1a8d62b6-e673-4dc5-9a27-a675032f7f61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_38acd1ff-f349-4415-b18e-9556528f03b1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_2e962f26-8b5d-479a-883e-c51857f2018f" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_38acd1ff-f349-4415-b18e-9556528f03b1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_38acd1ff-f349-4415-b18e-9556528f03b1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_38acd1ff-f349-4415-b18e-9556528f03b1" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_38acd1ff-f349-4415-b18e-9556528f03b1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8fff6851-7f86-4b1f-b480-69b56e01ddaa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_38acd1ff-f349-4415-b18e-9556528f03b1" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8fff6851-7f86-4b1f-b480-69b56e01ddaa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_SoftwareServicesLLCMember_130fc7c4-bdc2-4360-a15d-886db896b078" xlink:href="iiiv-20250630.xsd#iiiv_SoftwareServicesLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8fff6851-7f86-4b1f-b480-69b56e01ddaa" xlink:to="loc_iiiv_SoftwareServicesLLCMember_130fc7c4-bdc2-4360-a15d-886db896b078" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyDamagesSoughtValue_91609ffc-b6d7-4d0c-a54b-c333772f1c8b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyDamagesSoughtValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_ea30a174-d00a-4a21-8d42-67c83052bd44" xlink:to="loc_us-gaap_LossContingencyDamagesSoughtValue_91609ffc-b6d7-4d0c-a54b-c333772f1c8b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_4e3deaae-bf71-48b7-a3b3-241081bb437d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_ea30a174-d00a-4a21-8d42-67c83052bd44" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_4e3deaae-bf71-48b7-a3b3-241081bb437d" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/RELATEDPARTYTRANSACTIONSDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#RELATEDPARTYTRANSACTIONSDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/RELATEDPARTYTRANSACTIONSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_85f4668b-5bc6-48e7-bb97-37391b67df21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_85f4668b-5bc6-48e7-bb97-37391b67df21" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_824c6cfb-f914-4f9a-ba82-e2519e0326b3" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_85f4668b-5bc6-48e7-bb97-37391b67df21" xlink:to="loc_srt_CounterpartyNameAxis_824c6cfb-f914-4f9a-ba82-e2519e0326b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_824c6cfb-f914-4f9a-ba82-e2519e0326b3_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_824c6cfb-f914-4f9a-ba82-e2519e0326b3" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_824c6cfb-f914-4f9a-ba82-e2519e0326b3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_50cda902-9cc3-4751-93a5-615063765ea7" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_824c6cfb-f914-4f9a-ba82-e2519e0326b3" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_50cda902-9cc3-4751-93a5-615063765ea7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_ContinuingEquityOwnersMember_f82668c8-3598-4436-9ca5-271f8c4af5af" xlink:href="iiiv-20250630.xsd#iiiv_ContinuingEquityOwnersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_50cda902-9cc3-4751-93a5-615063765ea7" xlink:to="loc_iiiv_ContinuingEquityOwnersMember_f82668c8-3598-4436-9ca5-271f8c4af5af" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_b906b73b-6c02-4ea1-aec1-76a3bf7edb27" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_85f4668b-5bc6-48e7-bb97-37391b67df21" xlink:to="loc_us-gaap_StatementClassOfStockAxis_b906b73b-6c02-4ea1-aec1-76a3bf7edb27" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_b906b73b-6c02-4ea1-aec1-76a3bf7edb27_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_b906b73b-6c02-4ea1-aec1-76a3bf7edb27" xlink:to="loc_us-gaap_ClassOfStockDomain_b906b73b-6c02-4ea1-aec1-76a3bf7edb27_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_660a5ede-fd8e-4947-beea-66024a7a039e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_b906b73b-6c02-4ea1-aec1-76a3bf7edb27" xlink:to="loc_us-gaap_ClassOfStockDomain_660a5ede-fd8e-4947-beea-66024a7a039e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_03647df2-d7e4-4321-a727-019dd478e513" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_660a5ede-fd8e-4947-beea-66024a7a039e" xlink:to="loc_us-gaap_CommonClassBMember_03647df2-d7e4-4321-a727-019dd478e513" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_b5485e27-2f94-497d-9a9d-cbe8cf38e4ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_660a5ede-fd8e-4947-beea-66024a7a039e" xlink:to="loc_us-gaap_CommonClassAMember_b5485e27-2f94-497d-9a9d-cbe8cf38e4ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis_37bfed7e-0b11-4f39-b78d-43e0e10531d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_85f4668b-5bc6-48e7-bb97-37391b67df21" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis_37bfed7e-0b11-4f39-b78d-43e0e10531d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LegalEntityTypeOfCounterpartyDomain_37bfed7e-0b11-4f39-b78d-43e0e10531d4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LegalEntityTypeOfCounterpartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis_37bfed7e-0b11-4f39-b78d-43e0e10531d4" xlink:to="loc_us-gaap_LegalEntityTypeOfCounterpartyDomain_37bfed7e-0b11-4f39-b78d-43e0e10531d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LegalEntityTypeOfCounterpartyDomain_d9e3eb96-a38e-4850-82da-cf5501f967b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LegalEntityTypeOfCounterpartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis_37bfed7e-0b11-4f39-b78d-43e0e10531d4" xlink:to="loc_us-gaap_LegalEntityTypeOfCounterpartyDomain_d9e3eb96-a38e-4850-82da-cf5501f967b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_I3VerticalsLLCMember_bc2abd1e-b326-410b-8845-98821e0f030f" xlink:href="iiiv-20250630.xsd#iiiv_I3VerticalsLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LegalEntityTypeOfCounterpartyDomain_d9e3eb96-a38e-4850-82da-cf5501f967b0" xlink:to="loc_iiiv_I3VerticalsLLCMember_bc2abd1e-b326-410b-8845-98821e0f030f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_654face9-285c-4976-8308-10f04ba29d93" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_85f4668b-5bc6-48e7-bb97-37391b67df21" xlink:to="loc_dei_LegalEntityAxis_654face9-285c-4976-8308-10f04ba29d93" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_654face9-285c-4976-8308-10f04ba29d93_default" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_654face9-285c-4976-8308-10f04ba29d93" xlink:to="loc_dei_EntityDomain_654face9-285c-4976-8308-10f04ba29d93_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_2cd92f75-95d4-4f52-9e4c-cd1ea2a275ac" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_654face9-285c-4976-8308-10f04ba29d93" xlink:to="loc_dei_EntityDomain_2cd92f75-95d4-4f52-9e4c-cd1ea2a275ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_I3VerticalsLLCMember_c4fc84df-cd34-404a-b503-11e197061249" xlink:href="iiiv-20250630.xsd#iiiv_I3VerticalsLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_2cd92f75-95d4-4f52-9e4c-cd1ea2a275ac" xlink:to="loc_iiiv_I3VerticalsLLCMember_c4fc84df-cd34-404a-b503-11e197061249" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipAxis_35c97c62-dbb9-408c-a753-4bc6ad7054cf" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_85f4668b-5bc6-48e7-bb97-37391b67df21" xlink:to="loc_srt_OwnershipAxis_35c97c62-dbb9-408c-a753-4bc6ad7054cf" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_35c97c62-dbb9-408c-a753-4bc6ad7054cf_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_35c97c62-dbb9-408c-a753-4bc6ad7054cf" xlink:to="loc_srt_OwnershipDomain_35c97c62-dbb9-408c-a753-4bc6ad7054cf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_7a48c2fc-e329-4622-be43-744fbffc1a8a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_35c97c62-dbb9-408c-a753-4bc6ad7054cf" xlink:to="loc_srt_OwnershipDomain_7a48c2fc-e329-4622-be43-744fbffc1a8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_I3VerticalsLLCMember_76e710f5-300a-4265-95fe-8b646c514223" xlink:href="iiiv-20250630.xsd#iiiv_I3VerticalsLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_7a48c2fc-e329-4622-be43-744fbffc1a8a" xlink:to="loc_iiiv_I3VerticalsLLCMember_76e710f5-300a-4265-95fe-8b646c514223" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_2d93b56f-dadf-4149-a2f9-6083e305ef54" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_85f4668b-5bc6-48e7-bb97-37391b67df21" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_2d93b56f-dadf-4149-a2f9-6083e305ef54" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_2d93b56f-dadf-4149-a2f9-6083e305ef54_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_2d93b56f-dadf-4149-a2f9-6083e305ef54" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_2d93b56f-dadf-4149-a2f9-6083e305ef54_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_06660f73-fc65-4da8-b894-aa7238b3beec" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_2d93b56f-dadf-4149-a2f9-6083e305ef54" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_06660f73-fc65-4da8-b894-aa7238b3beec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_I3VerticalsLLCMember_68950441-24bf-4169-b550-9385383a3615" xlink:href="iiiv-20250630.xsd#iiiv_I3VerticalsLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_06660f73-fc65-4da8-b894-aa7238b3beec" xlink:to="loc_iiiv_I3VerticalsLLCMember_68950441-24bf-4169-b550-9385383a3615" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_TaxReceivableAgreementPercentOfTaxBenefitsPayableToContinuingEquityOwners_77dcd8bf-1c63-44e0-a229-126df910cf1d" xlink:href="iiiv-20250630.xsd#iiiv_TaxReceivableAgreementPercentOfTaxBenefitsPayableToContinuingEquityOwners"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_iiiv_TaxReceivableAgreementPercentOfTaxBenefitsPayableToContinuingEquityOwners_77dcd8bf-1c63-44e0-a229-126df910cf1d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_TaxBenefitDueToRelatedParties_ca4733ed-38cf-4487-a2dc-522c014961ea" xlink:href="iiiv-20250630.xsd#iiiv_TaxBenefitDueToRelatedParties"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_iiiv_TaxBenefitDueToRelatedParties_ca4733ed-38cf-4487-a2dc-522c014961ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesPurchaseOfInterestByParent_21f504b1-c96f-414b-b0e4-51e8b98afb4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesPurchaseOfInterestByParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesPurchaseOfInterestByParent_21f504b1-c96f-414b-b0e4-51e8b98afb4a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_552b1dcf-8922-4e43-b9be-e50b4d6ce440" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_552b1dcf-8922-4e43-b9be-e50b4d6ce440" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharePrice_e6fdd844-5eba-48c1-95a8-b3a36a34fb8b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_us-gaap_SharePrice_e6fdd844-5eba-48c1-95a8-b3a36a34fb8b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_NumberOfDaysVolumeWeightedAveragePrice_fabed77f-6314-4877-ac43-cf5d6614f49f" xlink:href="iiiv-20250630.xsd#iiiv_NumberOfDaysVolumeWeightedAveragePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_iiiv_NumberOfDaysVolumeWeightedAveragePrice_fabed77f-6314-4877-ac43-cf5d6614f49f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_RecapitalizationExchangeRatio_4d717da3-c6a4-4ede-a8ba-fe6bf18b98d1" xlink:href="iiiv-20250630.xsd#iiiv_RecapitalizationExchangeRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_iiiv_RecapitalizationExchangeRatio_4d717da3-c6a4-4ede-a8ba-fe6bf18b98d1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_5bc72f77-159b-486e-ac6a-2471396c621a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_5bc72f77-159b-486e-ac6a-2471396c621a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_ReverseRecapitalizationSharesRetired_163b1faf-2eec-4df5-a941-39359177c5d9" xlink:href="iiiv-20250630.xsd#iiiv_ReverseRecapitalizationSharesRetired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_iiiv_ReverseRecapitalizationSharesRetired_163b1faf-2eec-4df5-a941-39359177c5d9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_a1e5ddf8-ec15-4e2d-b749-9eb117bc4bd4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterestOwnershipPercentageByParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_a1e5ddf8-ec15-4e2d-b749-9eb117bc4bd4" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_SubsidiaryOwnershipIncreasePercentageParent_e621d263-0256-4e6e-a6ae-2d2b8d6bd508" xlink:href="iiiv-20250630.xsd#iiiv_SubsidiaryOwnershipIncreasePercentageParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_iiiv_SubsidiaryOwnershipIncreasePercentageParent_e621d263-0256-4e6e-a6ae-2d2b8d6bd508" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_dc1e7be2-6729-491d-8d5f-68d1e8cf8264" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_dc1e7be2-6729-491d-8d5f-68d1e8cf8264" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_EquityMethodInvestmentOwnershipPercentageDecreases_4d6e4660-26e7-4803-a377-a94ecc82d8c2" xlink:href="iiiv-20250630.xsd#iiiv_EquityMethodInvestmentOwnershipPercentageDecreases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_bc70c73f-2953-4449-a7ee-9e5d11ebdc6a" xlink:to="loc_iiiv_EquityMethodInvestmentOwnershipPercentageDecreases_4d6e4660-26e7-4803-a377-a94ecc82d8c2" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/NONCONTROLLINGINTERESTAdditionalInformationDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#NONCONTROLLINGINTERESTAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/NONCONTROLLINGINTERESTAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestLineItems_cd9e4a3d-a65a-4991-b601-f7af7e197094" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterestLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestTable_e005d686-b024-4fed-aeec-5906fc2a601b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterestTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MinorityInterestLineItems_cd9e4a3d-a65a-4991-b601-f7af7e197094" xlink:to="loc_us-gaap_MinorityInterestTable_e005d686-b024-4fed-aeec-5906fc2a601b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipAxis_07031fdf-9f1e-4d7c-85dd-8448857d5164" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MinorityInterestTable_e005d686-b024-4fed-aeec-5906fc2a601b" xlink:to="loc_srt_OwnershipAxis_07031fdf-9f1e-4d7c-85dd-8448857d5164" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_07031fdf-9f1e-4d7c-85dd-8448857d5164_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_07031fdf-9f1e-4d7c-85dd-8448857d5164" xlink:to="loc_srt_OwnershipDomain_07031fdf-9f1e-4d7c-85dd-8448857d5164_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_42cdf9e9-340f-4a72-9610-b95975e5ca3d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_07031fdf-9f1e-4d7c-85dd-8448857d5164" xlink:to="loc_srt_OwnershipDomain_42cdf9e9-340f-4a72-9610-b95975e5ca3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_I3VerticalsLLCMember_56efeb4d-7c7b-4075-b120-8213e1e7e890" xlink:href="iiiv-20250630.xsd#iiiv_I3VerticalsLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_42cdf9e9-340f-4a72-9610-b95975e5ca3d" xlink:to="loc_iiiv_I3VerticalsLLCMember_56efeb4d-7c7b-4075-b120-8213e1e7e890" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_NoncontrollingInterestCommonUnitOwnershipByParent_28d03edc-e029-437d-b03a-62ba490e103d" xlink:href="iiiv-20250630.xsd#iiiv_NoncontrollingInterestCommonUnitOwnershipByParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MinorityInterestLineItems_cd9e4a3d-a65a-4991-b601-f7af7e197094" xlink:to="loc_iiiv_NoncontrollingInterestCommonUnitOwnershipByParent_28d03edc-e029-437d-b03a-62ba490e103d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_bd84f868-5be2-4c62-8700-f0b9f5536d73" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterestOwnershipPercentageByParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MinorityInterestLineItems_cd9e4a3d-a65a-4991-b601-f7af7e197094" xlink:to="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_bd84f868-5be2-4c62-8700-f0b9f5536d73" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/EARNINGSPERSHAREContinuingOperationsDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#EARNINGSPERSHAREContinuingOperationsDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/EARNINGSPERSHAREContinuingOperationsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicLineItems_ba6e9d1e-f5a8-44bc-b666-95a265289ea8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasicLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_ba4034d7-5527-41a0-b01f-a8ff64349cb3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_ba6e9d1e-f5a8-44bc-b666-95a265289ea8" xlink:to="loc_us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_ba4034d7-5527-41a0-b01f-a8ff64349cb3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_04b30a74-4d3e-472e-ba4c-95fdba4da3d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_ba4034d7-5527-41a0-b01f-a8ff64349cb3" xlink:to="loc_us-gaap_StatementClassOfStockAxis_04b30a74-4d3e-472e-ba4c-95fdba4da3d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_04b30a74-4d3e-472e-ba4c-95fdba4da3d3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_04b30a74-4d3e-472e-ba4c-95fdba4da3d3" xlink:to="loc_us-gaap_ClassOfStockDomain_04b30a74-4d3e-472e-ba4c-95fdba4da3d3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_2a8637bc-3e2d-4c83-9f99-0103a8829400" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_04b30a74-4d3e-472e-ba4c-95fdba4da3d3" xlink:to="loc_us-gaap_ClassOfStockDomain_2a8637bc-3e2d-4c83-9f99-0103a8829400" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_419b641b-c5d8-4fa2-bb3d-a7a89715e7ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_2a8637bc-3e2d-4c83-9f99-0103a8829400" xlink:to="loc_us-gaap_CommonClassAMember_419b641b-c5d8-4fa2-bb3d-a7a89715e7ce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_7c5a7be2-b611-46bb-a837-5e2da0eede49" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementOperatingActivitiesSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_ba4034d7-5527-41a0-b01f-a8ff64349cb3" xlink:to="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_7c5a7be2-b611-46bb-a837-5e2da0eede49" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_7c5a7be2-b611-46bb-a837-5e2da0eede49_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_7c5a7be2-b611-46bb-a837-5e2da0eede49" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_7c5a7be2-b611-46bb-a837-5e2da0eede49_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_7858e499-a970-4885-8ba0-9b5a41fd0ce2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_7c5a7be2-b611-46bb-a837-5e2da0eede49" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_7858e499-a970-4885-8ba0-9b5a41fd0ce2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentContinuingOperationsMember_6cd0ef4c-176c-4e40-ac72-28da69a10334" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentContinuingOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentOperatingActivitiesDomain_7858e499-a970-4885-8ba0-9b5a41fd0ce2" xlink:to="loc_us-gaap_SegmentContinuingOperationsMember_6cd0ef4c-176c-4e40-ac72-28da69a10334" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicAbstract_ac587c8b-7586-4e28-adaf-e2208f9c5894" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasicAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_ba6e9d1e-f5a8-44bc-b666-95a265289ea8" xlink:to="loc_us-gaap_EarningsPerShareBasicAbstract_ac587c8b-7586-4e28-adaf-e2208f9c5894" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest_cf83067c-2fd5-4764-8a5a-8b29deb2be8c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_ac587c8b-7586-4e28-adaf-e2208f9c5894" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest_cf83067c-2fd5-4764-8a5a-8b29deb2be8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsAttributableToNoncontrollingEntity_2938ef44-9653-422b-88a9-24178d936f8c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsAttributableToNoncontrollingEntity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_ac587c8b-7586-4e28-adaf-e2208f9c5894" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsAttributableToNoncontrollingEntity_2938ef44-9653-422b-88a9-24178d936f8c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossFromContinuingOperationsAvailableToCommonShareholdersBasic_a5d79ecf-9387-45e0-a07e-3236af231dd0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossFromContinuingOperationsAvailableToCommonShareholdersBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_ac587c8b-7586-4e28-adaf-e2208f9c5894" xlink:to="loc_us-gaap_NetIncomeLossFromContinuingOperationsAvailableToCommonShareholdersBasic_a5d79ecf-9387-45e0-a07e-3236af231dd0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_6eced739-dbf6-45bd-9ae6-94dc42078ac7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_ac587c8b-7586-4e28-adaf-e2208f9c5894" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_6eced739-dbf6-45bd-9ae6-94dc42078ac7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsPerBasicShare_b8129522-997a-428a-bc5b-f4d95741c9d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsPerBasicShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_ac587c8b-7586-4e28-adaf-e2208f9c5894" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsPerBasicShare_b8129522-997a-428a-bc5b-f4d95741c9d6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDilutedAbstract_84c53a03-3ad0-4d8b-9e2e-65630022e7e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_ba6e9d1e-f5a8-44bc-b666-95a265289ea8" xlink:to="loc_us-gaap_EarningsPerShareDilutedAbstract_84c53a03-3ad0-4d8b-9e2e-65630022e7e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossFromContinuingOperationsAvailableToCommonShareholdersBasic_1487a159-26d0-4601-823a-1cb900ebb31d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossFromContinuingOperationsAvailableToCommonShareholdersBasic"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DilutiveSecurities_8f2de85f-3e2b-4105-a41a-0eb9e4068f2e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DilutiveSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_84c53a03-3ad0-4d8b-9e2e-65630022e7e0" xlink:to="loc_us-gaap_DilutiveSecurities_8f2de85f-3e2b-4105-a41a-0eb9e4068f2e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossFromContinuingOperationsAvailableToCommonShareholdersDiluted_3ba31433-740c-4833-8804-d7ab7b5615be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossFromContinuingOperationsAvailableToCommonShareholdersDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_84c53a03-3ad0-4d8b-9e2e-65630022e7e0" xlink:to="loc_us-gaap_NetIncomeLossFromContinuingOperationsAvailableToCommonShareholdersDiluted_3ba31433-740c-4833-8804-d7ab7b5615be" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_5c16206d-59f4-49a8-b67b-eb10845672fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment_fdd0900d-f148-4813-bde6-a4e1f0bb8965" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_84c53a03-3ad0-4d8b-9e2e-65630022e7e0" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment_fdd0900d-f148-4813-bde6-a4e1f0bb8965" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_454645f5-348d-478c-a742-cfb37e6f59e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_84c53a03-3ad0-4d8b-9e2e-65630022e7e0" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_454645f5-348d-478c-a742-cfb37e6f59e9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare_389fbf00-9a94-4cf4-8f2a-7915a6720fc1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_84c53a03-3ad0-4d8b-9e2e-65630022e7e0" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare_389fbf00-9a94-4cf4-8f2a-7915a6720fc1" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/EARNINGSPERSHAREAntidilutiveSecuritiesDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#EARNINGSPERSHAREAntidilutiveSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/EARNINGSPERSHAREAntidilutiveSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_fe597023-a191-4690-a6b9-e1a9587c01ca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_7f3fa791-b466-483d-9792-465bf2a6e4f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_fe597023-a191-4690-a6b9-e1a9587c01ca" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_7f3fa791-b466-483d-9792-465bf2a6e4f3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_da9acf88-06c6-40a1-98d4-ecfb73a6592b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_7f3fa791-b466-483d-9792-465bf2a6e4f3" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_da9acf88-06c6-40a1-98d4-ecfb73a6592b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_da9acf88-06c6-40a1-98d4-ecfb73a6592b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_da9acf88-06c6-40a1-98d4-ecfb73a6592b" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_da9acf88-06c6-40a1-98d4-ecfb73a6592b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_82c4e4e2-0a95-4f1f-ae80-418af7b8f59c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_da9acf88-06c6-40a1-98d4-ecfb73a6592b" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_82c4e4e2-0a95-4f1f-ae80-418af7b8f59c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_d6095fd6-fbed-43d2-94af-965f102b2210" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_82c4e4e2-0a95-4f1f-ae80-418af7b8f59c" xlink:to="loc_us-gaap_CommonClassBMember_d6095fd6-fbed-43d2-94af-965f102b2210" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_OutoftheMoneyStockOptionsMember_46319523-07db-4bfd-b697-5351340f3ff7" xlink:href="iiiv-20250630.xsd#iiiv_OutoftheMoneyStockOptionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_82c4e4e2-0a95-4f1f-ae80-418af7b8f59c" xlink:to="loc_iiiv_OutoftheMoneyStockOptionsMember_46319523-07db-4bfd-b697-5351340f3ff7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_EmployeeStockOptionsAndRestrictedStockUnitsRSUsMember_d1f94840-1268-47e2-b51a-ba41a59f513a" xlink:href="iiiv-20250630.xsd#iiiv_EmployeeStockOptionsAndRestrictedStockUnitsRSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_82c4e4e2-0a95-4f1f-ae80-418af7b8f59c" xlink:to="loc_iiiv_EmployeeStockOptionsAndRestrictedStockUnitsRSUsMember_d1f94840-1268-47e2-b51a-ba41a59f513a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_8321ab3b-9690-4447-83b3-b5c910334902" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementOperatingActivitiesSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_7f3fa791-b466-483d-9792-465bf2a6e4f3" xlink:to="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_8321ab3b-9690-4447-83b3-b5c910334902" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_8321ab3b-9690-4447-83b3-b5c910334902_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_8321ab3b-9690-4447-83b3-b5c910334902" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_8321ab3b-9690-4447-83b3-b5c910334902_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_7f7c8d36-2434-43c2-85d8-bc5d66e5dd06" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_8321ab3b-9690-4447-83b3-b5c910334902" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_7f7c8d36-2434-43c2-85d8-bc5d66e5dd06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentContinuingOperationsMember_518d95c9-6857-4236-8780-e7966241617a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentContinuingOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentOperatingActivitiesDomain_7f7c8d36-2434-43c2-85d8-bc5d66e5dd06" xlink:to="loc_us-gaap_SegmentContinuingOperationsMember_518d95c9-6857-4236-8780-e7966241617a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDiscontinuedOperationsMember_caf85ed6-cbfd-47f9-bd6f-7c0b3e920c18" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDiscontinuedOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentOperatingActivitiesDomain_7f7c8d36-2434-43c2-85d8-bc5d66e5dd06" xlink:to="loc_us-gaap_SegmentDiscontinuedOperationsMember_caf85ed6-cbfd-47f9-bd6f-7c0b3e920c18" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_e85877d1-929a-4e1c-8a10-ff9dd2514b27" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_fe597023-a191-4690-a6b9-e1a9587c01ca" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_e85877d1-929a-4e1c-8a10-ff9dd2514b27" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/EARNINGSPERSHAREDiscontinuingOperationsDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#EARNINGSPERSHAREDiscontinuingOperationsDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/EARNINGSPERSHAREDiscontinuingOperationsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicLineItems_83e565e5-c2f5-4e10-b235-f8c61d53d29c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasicLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_55fb8340-374e-49b9-a5ab-ca722f9b8c14" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_83e565e5-c2f5-4e10-b235-f8c61d53d29c" xlink:to="loc_us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_55fb8340-374e-49b9-a5ab-ca722f9b8c14" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_20ae5213-db67-458e-b1ec-e3c8015a53bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementOperatingActivitiesSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_55fb8340-374e-49b9-a5ab-ca722f9b8c14" xlink:to="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_20ae5213-db67-458e-b1ec-e3c8015a53bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_20ae5213-db67-458e-b1ec-e3c8015a53bf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_20ae5213-db67-458e-b1ec-e3c8015a53bf" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_20ae5213-db67-458e-b1ec-e3c8015a53bf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_b59922bb-b759-415d-a95c-30f6ce59238d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_20ae5213-db67-458e-b1ec-e3c8015a53bf" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_b59922bb-b759-415d-a95c-30f6ce59238d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDiscontinuedOperationsMember_524dae8a-5e5a-4b7a-8dc1-b67b8b8d1668" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDiscontinuedOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentOperatingActivitiesDomain_b59922bb-b759-415d-a95c-30f6ce59238d" xlink:to="loc_us-gaap_SegmentDiscontinuedOperationsMember_524dae8a-5e5a-4b7a-8dc1-b67b8b8d1668" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicAbstract_cbfb5928-b595-4e5a-adca-e8f19444524f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasicAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_83e565e5-c2f5-4e10-b235-f8c61d53d29c" xlink:to="loc_us-gaap_EarningsPerShareBasicAbstract_cbfb5928-b595-4e5a-adca-e8f19444524f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_8cd59278-2d6a-4a00-91f4-bc0754296eff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_cbfb5928-b595-4e5a-adca-e8f19444524f" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_8cd59278-2d6a-4a00-91f4-bc0754296eff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToNoncontrollingInterest_ae73e8e3-fca7-4ad4-8717-da54d427fad1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_cbfb5928-b595-4e5a-adca-e8f19444524f" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToNoncontrollingInterest_ae73e8e3-fca7-4ad4-8717-da54d427fad1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossFromDiscontinuedOperationsAvailableToCommonShareholdersBasic_890d0bef-13d2-4bed-aa83-429d57976f60" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossFromDiscontinuedOperationsAvailableToCommonShareholdersBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_cbfb5928-b595-4e5a-adca-e8f19444524f" xlink:to="loc_us-gaap_NetIncomeLossFromDiscontinuedOperationsAvailableToCommonShareholdersBasic_890d0bef-13d2-4bed-aa83-429d57976f60" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_d982308c-1ab6-4f2c-9e15-e33e7d6a7c86" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_cbfb5928-b595-4e5a-adca-e8f19444524f" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_d982308c-1ab6-4f2c-9e15-e33e7d6a7c86" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare_a0cd5ffe-12c1-43b9-8cc8-9d30ffb4ef9e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_cbfb5928-b595-4e5a-adca-e8f19444524f" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare_a0cd5ffe-12c1-43b9-8cc8-9d30ffb4ef9e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDilutedAbstract_dd487ff5-abd6-49b5-8e56-6e346035c06f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_83e565e5-c2f5-4e10-b235-f8c61d53d29c" xlink:to="loc_us-gaap_EarningsPerShareDilutedAbstract_dd487ff5-abd6-49b5-8e56-6e346035c06f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossFromDiscontinuedOperationsAvailableToCommonShareholdersBasic_f4170537-d93c-4690-9599-2ea738760476" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossFromDiscontinuedOperationsAvailableToCommonShareholdersBasic"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_ReallocationOfNetIncomeAssumingConversionOfCommonUnitsEffectOnBasicEarningsPerShare_22293483-8ab0-4725-8be0-090888580be0" xlink:href="iiiv-20250630.xsd#iiiv_ReallocationOfNetIncomeAssumingConversionOfCommonUnitsEffectOnBasicEarningsPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_dd487ff5-abd6-49b5-8e56-6e346035c06f" xlink:to="loc_iiiv_ReallocationOfNetIncomeAssumingConversionOfCommonUnitsEffectOnBasicEarningsPerShare_22293483-8ab0-4725-8be0-090888580be0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossFromDiscontinuedOperationsAvailableToCommonShareholdersDiluted_be6a0f16-7c70-4571-bd99-db4caded79a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossFromDiscontinuedOperationsAvailableToCommonShareholdersDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_dd487ff5-abd6-49b5-8e56-6e346035c06f" xlink:to="loc_us-gaap_NetIncomeLossFromDiscontinuedOperationsAvailableToCommonShareholdersDiluted_be6a0f16-7c70-4571-bd99-db4caded79a1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_b57fc959-68d3-4598-9410-34bcd29d7dc7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment_866e9671-745d-444d-adf8-32c7bb312e99" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_dd487ff5-abd6-49b5-8e56-6e346035c06f" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment_866e9671-745d-444d-adf8-32c7bb312e99" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_90ed3403-81a0-4d46-b1de-5d83a48837c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_dd487ff5-abd6-49b5-8e56-6e346035c06f" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_90ed3403-81a0-4d46-b1de-5d83a48837c7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare_ce97667d-a52a-4c04-8734-b69f5381a5c3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_dd487ff5-abd6-49b5-8e56-6e346035c06f" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare_ce97667d-a52a-4c04-8734-b69f5381a5c3" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/EARNINGSPERSHARENarrativeDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#EARNINGSPERSHARENarrativeDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/EARNINGSPERSHARENarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_EquityLineItems_e72c483c-a399-472b-8974-65c62fed3024" xlink:href="iiiv-20250630.xsd#iiiv_EquityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_iiiv_EquityTable_a67de33d-759c-4a85-b40c-4689aec8dccf" xlink:href="iiiv-20250630.xsd#iiiv_EquityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_iiiv_EquityLineItems_e72c483c-a399-472b-8974-65c62fed3024" xlink:to="loc_iiiv_EquityTable_a67de33d-759c-4a85-b40c-4689aec8dccf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_d4b8c7e1-087e-4b0e-a16b-d99d13984675" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_iiiv_EquityTable_a67de33d-759c-4a85-b40c-4689aec8dccf" xlink:to="loc_us-gaap_StatementClassOfStockAxis_d4b8c7e1-087e-4b0e-a16b-d99d13984675" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_d4b8c7e1-087e-4b0e-a16b-d99d13984675_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_d4b8c7e1-087e-4b0e-a16b-d99d13984675" xlink:to="loc_us-gaap_ClassOfStockDomain_d4b8c7e1-087e-4b0e-a16b-d99d13984675_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_fa6256de-505e-4d02-a67a-446c861de779" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_d4b8c7e1-087e-4b0e-a16b-d99d13984675" xlink:to="loc_us-gaap_ClassOfStockDomain_fa6256de-505e-4d02-a67a-446c861de779" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_1287a6f3-488b-468c-a4b0-ae0ea5a02a47" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_fa6256de-505e-4d02-a67a-446c861de779" xlink:to="loc_us-gaap_CommonClassAMember_1287a6f3-488b-468c-a4b0-ae0ea5a02a47" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_ac2a043d-a80b-4ca0-8a90-c3701e275949" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_EquityLineItems_e72c483c-a399-472b-8974-65c62fed3024" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_ac2a043d-a80b-4ca0-8a90-c3701e275949" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_e6fd47ff-c545-430f-9ca0-41392d8a6f25" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_iiiv_EquityLineItems_e72c483c-a399-472b-8974-65c62fed3024" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_e6fd47ff-c545-430f-9ca0-41392d8a6f25" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://i3verticals.com/role/SUBSEQUENTEVENTSDetails" xlink:type="simple" xlink:href="iiiv-20250630.xsd#SUBSEQUENTEVENTSDetails"/>
  <link:definitionLink xlink:role="http://i3verticals.com/role/SUBSEQUENTEVENTSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_0653dce6-a57a-4e76-bbca-e2110e61ee0a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_82921a6f-6e94-4ff1-85df-3ecc8d97c5f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_0653dce6-a57a-4e76-bbca-e2110e61ee0a" xlink:to="loc_us-gaap_SubsequentEventTable_82921a6f-6e94-4ff1-85df-3ecc8d97c5f7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_3fd594f5-7398-4000-aaab-784d12db642b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_82921a6f-6e94-4ff1-85df-3ecc8d97c5f7" xlink:to="loc_us-gaap_StatementClassOfStockAxis_3fd594f5-7398-4000-aaab-784d12db642b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_3fd594f5-7398-4000-aaab-784d12db642b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_3fd594f5-7398-4000-aaab-784d12db642b" xlink:to="loc_us-gaap_ClassOfStockDomain_3fd594f5-7398-4000-aaab-784d12db642b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_931293e4-2bbb-445c-8870-01eed713d0a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_3fd594f5-7398-4000-aaab-784d12db642b" xlink:to="loc_us-gaap_ClassOfStockDomain_931293e4-2bbb-445c-8870-01eed713d0a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_649339eb-1da7-4c1b-ac07-b4e03e047b1a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_931293e4-2bbb-445c-8870-01eed713d0a0" xlink:to="loc_us-gaap_CommonClassAMember_649339eb-1da7-4c1b-ac07-b4e03e047b1a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_78c87160-d268-4586-a26c-c6c8f6ffb533" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_82921a6f-6e94-4ff1-85df-3ecc8d97c5f7" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_78c87160-d268-4586-a26c-c6c8f6ffb533" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_78c87160-d268-4586-a26c-c6c8f6ffb533_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_78c87160-d268-4586-a26c-c6c8f6ffb533" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_78c87160-d268-4586-a26c-c6c8f6ffb533_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_97ea7379-1eba-49ce-8077-193473f759d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_78c87160-d268-4586-a26c-c6c8f6ffb533" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_97ea7379-1eba-49ce-8077-193473f759d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_c4e41466-5422-4c5d-a79d-73948fad3fdd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_97ea7379-1eba-49ce-8077-193473f759d0" xlink:to="loc_us-gaap_SubsequentEventMember_c4e41466-5422-4c5d-a79d-73948fad3fdd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StockRepurchaseProgramAuthorizedAmount1_74c8272f-873f-4dca-998f-faa61e8ead75" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_0653dce6-a57a-4e76-bbca-e2110e61ee0a" xlink:to="loc_srt_StockRepurchaseProgramAuthorizedAmount1_74c8272f-873f-4dca-998f-faa61e8ead75" xlink:type="arc" order="0"/>
  </link:definitionLink>
</link:linkbase>
