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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2019-05-03</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000098362</issuerCik>
        <issuerName>TIMKEN CO</issuerName>
        <issuerTradingSymbol>TKR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001598539</rptOwnerCik>
            <rptOwnerName>Fracassa Philip D.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>4500 MT. PLEASANT ST. NW</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NORTH CANTON</rptOwnerCity>
            <rptOwnerState>OH</rptOwnerState>
            <rptOwnerZipCode>44720</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>EVP &amp; CFO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <transactionDate>
                <value>2019-05-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>I</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1875</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>52.16</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1875</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>401(k)</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">On June 26, 2018, the Company announced that effective January 1, 2019 the primary U.S. Company sponsored 401(k) plan would no longer allow contributions to be made to the employer stock fund to align with industry trends to remove investment in the employer's stock as an option in a company sponsored 401(k) plan.  All participants in this plan, including the named officer, are required to transfer remaining funds in the employer stock fund to other fund options by December 31, 2022.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Philip D. Fracassa</signatureName>
        <signatureDate>2019-05-07</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2019-05-03
0
0000098362
TIMKEN CO
TKR
0001598539
Fracassa Philip D.
4500 MT. PLEASANT ST. NW
NORTH CANTON
OH
44720
0
1
0
0
EVP & CFO
Common Stock
2019-05-03
4
I
0
1875
52.16
D
1875
I
401(k)
On June 26, 2018, the Company announced that effective January 1, 2019 the primary U.S. Company sponsored 401(k) plan would no longer allow contributions to be made to the employer stock fund to align with industry trends to remove investment in the employer's stock as an option in a company sponsored 401(k) plan.  All participants in this plan, including the named officer, are required to transfer remaining funds in the employer stock fund to other fund options by December 31, 2022.
/s/ Philip D. Fracassa
2019-05-07