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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2018-09-07</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, CFO &amp; Treasurer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2018-09-07</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>34295</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>34295</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Represents restricted share units that will vest in three approximately equal increments on September 7, 2019, September 7, 2020, and September 7, 2021 and settle in shares of Common Stock on a one-to-one basis, contingent upon continued employment through the vesting date.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Lisa Kamibayashi as Attorney-in-Fact for Ravi Mallela</signatureName>
        <signatureDate>2018-09-11</signatureDate>
    </ownerSignature>
</ownershipDocument>
