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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:01a086af-3355-7041-a6f9-038a34516df1,g:6e81ef1f-3aa3-4f23-8381-308db80d626d-->
<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://www.firstib.com/role/Cover" xlink:type="simple" xlink:href="inbk-20260910.xsd#Cover"/>
  <link:definitionLink xlink:role="http://www.firstib.com/role/Cover" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_01a086af-3355-7b85-a757-271b75865949" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_01a086af-3355-7b85-a757-271b75865949" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_01a086af-3355-7e8e-82a6-9cad30d82792" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_01a086af-3355-7b85-a757-271b75865949" xlink:to="loc_us-gaap_StatementClassOfStockAxis_01a086af-3355-7e8e-82a6-9cad30d82792" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_01a086af-3355-7e8e-82a6-9cad30d82792_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_01a086af-3355-7e8e-82a6-9cad30d82792" xlink:to="loc_us-gaap_ClassOfStockDomain_01a086af-3355-7e8e-82a6-9cad30d82792_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_01a086af-3355-710b-8742-aa5371568d41" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_01a086af-3355-7e8e-82a6-9cad30d82792" xlink:to="loc_us-gaap_ClassOfStockDomain_01a086af-3355-710b-8742-aa5371568d41" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a086af-3355-7b22-8dc7-fb24489c42ba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_01a086af-3355-710b-8742-aa5371568d41" xlink:to="loc_us-gaap_CommonStockMember_01a086af-3355-7b22-8dc7-fb24489c42ba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_inbk_A60FixedToFloatingSubordinatedNotesDue2029Member_01a086af-3355-7480-ac21-da98f2a700fb" xlink:href="inbk-20260910.xsd#inbk_A60FixedToFloatingSubordinatedNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_01a086af-3355-710b-8742-aa5371568d41" xlink:to="loc_inbk_A60FixedToFloatingSubordinatedNotesDue2029Member_01a086af-3355-7480-ac21-da98f2a700fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_01a086af-3355-7bbb-8632-7de2173dfdc1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_DocumentType_01a086af-3355-7bbb-8632-7de2173dfdc1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_01a086af-3355-7ff3-8bc2-a92de4af0df1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_DocumentPeriodEndDate_01a086af-3355-7ff3-8bc2-a92de4af0df1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_01a086af-3355-7fbb-8e99-d851cfa64135" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_EntityRegistrantName_01a086af-3355-7fbb-8e99-d851cfa64135" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_01a086af-3355-794b-891f-fb8374311a79" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_EntityIncorporationStateCountryCode_01a086af-3355-794b-891f-fb8374311a79" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_01a086af-3355-71fa-9081-9f057c3153df" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_EntityFileNumber_01a086af-3355-71fa-9081-9f057c3153df" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_01a086af-3355-7e44-8b89-0869155c3940" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_EntityTaxIdentificationNumber_01a086af-3355-7e44-8b89-0869155c3940" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_01a086af-3355-7ac6-9cf0-69730fe6410f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_EntityAddressAddressLine1_01a086af-3355-7ac6-9cf0-69730fe6410f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_01a086af-3355-7629-8961-65efccdb7d0d" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_EntityAddressCityOrTown_01a086af-3355-7629-8961-65efccdb7d0d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_01a086af-3355-7755-a40f-633754086edd" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_EntityAddressStateOrProvince_01a086af-3355-7755-a40f-633754086edd" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_01a086af-3355-7bb7-8768-9fa1c8d3645f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_EntityAddressPostalZipCode_01a086af-3355-7bb7-8768-9fa1c8d3645f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_01a086af-3355-7924-b191-4cdefef9e918" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_CityAreaCode_01a086af-3355-7924-b191-4cdefef9e918" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_01a086af-3355-7185-8780-4be7e950aa5a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_LocalPhoneNumber_01a086af-3355-7185-8780-4be7e950aa5a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_WrittenCommunications_01a086af-3355-78d2-b52d-455b594896a0" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_WrittenCommunications"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_WrittenCommunications_01a086af-3355-78d2-b52d-455b594896a0" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SolicitingMaterial_01a086af-3355-76d2-9d84-0a78adc617e1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SolicitingMaterial"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_SolicitingMaterial_01a086af-3355-76d2-9d84-0a78adc617e1" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementTenderOffer_01a086af-3355-7e66-9935-2bed0988c05b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_PreCommencementTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_PreCommencementTenderOffer_01a086af-3355-7e66-9935-2bed0988c05b" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementIssuerTenderOffer_01a086af-3355-7fd6-82ec-b393bcfcaf23" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_PreCommencementIssuerTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_PreCommencementIssuerTenderOffer_01a086af-3355-7fd6-82ec-b393bcfcaf23" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_01a086af-3355-7650-b139-deb5417939df" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_Security12bTitle_01a086af-3355-7650-b139-deb5417939df" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_01a086af-3355-765b-bd62-f2ed463102a0" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_TradingSymbol_01a086af-3355-765b-bd62-f2ed463102a0" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_01a086af-3355-7222-8bd9-86656e760b27" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_SecurityExchangeName_01a086af-3355-7222-8bd9-86656e760b27" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_01a086af-3355-77b4-8dea-4bb911a59233" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_EntityEmergingGrowthCompany_01a086af-3355-77b4-8dea-4bb911a59233" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_01a086af-3355-788b-b972-1ea288f237b4" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_EntityCentralIndexKey_01a086af-3355-788b-b972-1ea288f237b4" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_01a086af-3355-7436-b641-83988b29a89f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a086af-3355-75a3-ba65-ceabad5974f6" xlink:to="loc_dei_AmendmentFlag_01a086af-3355-7436-b641-83988b29a89f" xlink:type="arc" order="21"/>
  </link:definitionLink>
</link:linkbase>
