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

    <schemaVersion>X0508</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2025-12-10</periodOfReport>

    <issuer>
        <issuerCik>0001085913</issuerCik>
        <issuerName>FARMERS &amp; MERCHANTS BANCORP</issuerName>
        <issuerTradingSymbol>FMCB</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001695102</rptOwnerCik>
            <rptOwnerName>Olson Bart R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>FARMERS &amp; MERCHANTS BANCORP</rptOwnerStreet1>
            <rptOwnerStreet2>121 W. PINE STREET</rptOwnerStreet2>
            <rptOwnerCity>LODI</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>95240</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>false</isDirector>
            <isOfficer>true</isOfficer>
            <isTenPercentOwner>false</isTenPercentOwner>
            <isOther>false</isOther>
            <officerTitle>Executive Vice President</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2025-12-10</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>false</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>690</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>690</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held in family trust</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2422</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Shares previously held indirectly through the Company's Non-Qualified Executive Retirement Plans, which, in connection with the liquidation and distribution of the Plans, were distributed by the Trustees of the Plan to Reporting Person's family trust on December 10, 2025, for no consideration.  The Reporting Person is a trustee of the trust, and the Reporting Person and member of his immediate family are the sole beneficiaries of the trust.  The Reporting Person remains the beneficial owner of the securities held by the trust.</footnote>
        <footnote id="F2">Includes, as previously reported, grants on February 3, 2025 to Reporting Person of a RSA of 1,840 shares of Issuer's common stock, which will vest ratably over a two (2) year term; and a special one-time RSA of 582 shares of Issuer's common stock without voting rights, which will vest ratably over a three (3) year term.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Olson Bart R</signatureName>
        <signatureDate>2025-12-18</signatureDate>
    </ownerSignature>
</ownershipDocument>
