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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-04-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000713671</issuerCik>
        <issuerName>DNB FINANCIAL CORP /PA/</issuerName>
        <issuerTradingSymbol>DNBF</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001353138</rptOwnerCik>
            <rptOwnerName>Hartmann Richard J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>5175 PROVIDENCE ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>WALLINGFORD</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19086</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>EVP/Retail</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>DNB Common Stock,$1 par value,stock option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>6.93</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2010-04-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>6.93</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2014-04-23</value>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2017-04-23</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>DNB Common Stock, $1 par value</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3152</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Options to purchase common stock under DNB 1995 Stock Option Plan.Options have a 7 year term.Grantee first entitled to exercise on the earlier of 4 years from the Grant Date,death,termination of employment on account of disability,a change in control,or retirement at or after age 65.If Grantee's employment terminates prior to the vesting date,Grantee shall forfeit all rights under it.Grantee is subject to a 2 year holding period at the time of exercise, but may sell shares to pay withholding taxes. Grantee may not exercise the options until the market value of the outstanding common shares of the Registrant reaches 115% of the exercise price.Grantee's entitlement to Award Shares shall be subject to the further conditions pursuant to Section 111(b)(3)(D) of EESA. During any period that the Grantee is subject to applicable limits on executive compensation under TARP, the Grantee's rights under the award agreement are only enforceable to the extent permitted under TARP.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Gerald F Sopp for Richard Hartmann</signatureName>
        <signatureDate>2010-05-12</signatureDate>
    </ownerSignature>
</ownershipDocument>
