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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2020-05-06</periodOfReport>

    <issuer>
        <issuerCik>0000055135</issuerCik>
        <issuerName>KELLY SERVICES INC</issuerName>
        <issuerTradingSymbol>KELYA</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001305641</rptOwnerCik>
            <rptOwnerName>Parfet Donald R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O 350 E MICHIGAN AVENUE STE 500</rptOwnerStreet1>
            <rptOwnerStreet2>APJOHN GROUP LLC</rptOwnerStreet2>
            <rptOwnerCity>KALAMAZOO</rptOwnerCity>
            <rptOwnerState>MI</rptOwnerState>
            <rptOwnerZipCode>49007</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock, Par Value $1</value>
            </securityTitle>
            <transactionDate>
                <value>2020-05-06</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>12880</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>12.81</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>46555</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Stock granted under the terms of the Equity Incentive Plan. The award represents a portion of the annual retainer paid to covered  members of Kelly Services, Inc. Board of Directors. The stock was valued at close of market on 5/6/2020 at $12.81 per share.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Deborah M. Emerson, attorney-in-fact for Mr. Parfet</signatureName>
        <signatureDate>2020-05-07</signatureDate>
    </ownerSignature>
</ownershipDocument>
