<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0609</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2026-04-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001709442</issuerCik>
        <issuerName>FIRSTSUN CAPITAL BANCORP</issuerName>
        <issuerTradingSymbol>FSUN</issuerTradingSymbol>
        <issuerForeignTradingSymbol></issuerForeignTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001697577</rptOwnerCik>
            <rptOwnerName>MACKOVAK BENJAMIN</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerNonUSAddressFlag>false</rptOwnerNonUSAddressFlag>
            <rptOwnerStreet1>1400 16TH STREET</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 250</rptOwnerStreet2>
            <rptOwnerCity>DENVER</rptOwnerCity>
            <rptOwnerState>CO</rptOwnerState>
            <rptOwnerZipCode>80202</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, $0.0001 par value</value>
            </securityTitle>
            <transactionDate>
                <value>2026-04-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2140</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2140</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, $0.0001 par value</value>
            </securityTitle>
            <transactionDate>
                <value>2026-04-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1151160</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1151160</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Owned directly by Strategic Value Investors LP</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Received in exchange for 13,308 unvested restricted stock units (&quot;RSUs&quot;) of First Foundation Inc. (&quot;First Foundation&quot;) that were assumed by FirstSun Capital Bancorp (&quot;FirstSun&quot;) and replaced with RSUs of FirstSun, in connection with the merger of First Foundation with and into FirstSun. Each unvested RSU of First Foundation was converted into an RSU of FirstSun with respect to FirstSun common stock, based on the exchange ratio in the merger of 0.16083 of a share of FirstSun common stock for each share of First Foundation common stock.</footnote>
        <footnote id="F2">Received in exchange for (a) 6,768,343 shares of First Foundation common stock and (b) 389.28296 shares of First Foundation Series C non-voting common equity equivalent stock (the &quot;Series C stock&quot;), each in connection with the merger. Each share of Series C stock converted into the right to receive a number of shares of FirstSun common stock equal to the number of shares of First Foundation common stock that such share was convertible into immediately prior to the effective time of the merger, multiplied by the exchange ratio of 0.16083.  Each share of First Foundation common stock converted into the right to receive 0.16083 of a share of FirstSun common stock, with cash paid in lieu of fractional shares.  On the last trading day before the effective date of the merger, the closing price of First Foundation common stock was $5.90 per share, and the closing price of FirstSun common stock was $36.46 per share.</footnote>
        <footnote id="F3">Owned directly by Strategic Value Investors LP. The reporting person, solely by virtue of his position as a managing member of Strategic Value Bank Partners LLC, which serves as the general partner of Strategic Value Investors LP, may be deemed to beneficially own the shares owned by Strategic Value Investors LP for purposes of Section 16. The reporting person expressly disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Kelly C. Rackley, by power of attorney</signatureName>
        <signatureDate>2026-04-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
