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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2019-09-06</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000036377</issuerCik>
        <issuerName>FIRST HAWAIIAN, INC.</issuerName>
        <issuerTradingSymbol>FHB</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001751947</rptOwnerCik>
            <rptOwnerName>Mallela Ravi</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O FIRST HAWAIIAN, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>999 BISHOP STREET, 8TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>HONOLULU</rptOwnerCity>
            <rptOwnerState>HI</rptOwnerState>
            <rptOwnerZipCode>96813</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Exec VP &amp; CFO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2019-09-06</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>3859</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>25.64</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>42825</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Represents shares of the Issuer's common stock withheld to satisfy withholding obligations in connection with the delivery of the Issuer's common stock underlying the restricted share units, previously reported on Form 4 filed on September 11, 2018, that vested on September 7, 2019.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Lisa Kamibayashi as Attorney-in-Fact for Ravi Mallela</signatureName>
        <signatureDate>2019-09-09</signatureDate>
    </ownerSignature>
</ownershipDocument>
