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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-05-02</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000100726</issuerCik>
        <issuerName>UNIFI INC</issuerName>
        <issuerTradingSymbol>UFI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001259388</rptOwnerCik>
            <rptOwnerName>ARMFIELD WILLIAM J IV</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>300 N GREENE ST SUITE 2190</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>GREENSBORO</rptOwnerCity>
            <rptOwnerState>NC</rptOwnerState>
            <rptOwnerZipCode>27401</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>COMMON STOCK</value>
            </securityTitle>
            <transactionDate>
                <value>2013-05-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1326</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>19.65</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>290699</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>COMMON STOCK</value>
            </securityTitle>
            <transactionDate>
                <value>2013-05-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>3500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>19.68</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9639</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>BY WIFE</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The Reporting Person executed a trade order through a broker-dealer which resulted in multiple same-day same-way open market purchases, with the prices ranging from $19.05 to $19.84/share.  The Reporting Person has reported these purchases on an aggregate basis using the weighted average price for the transactions and will provide detailed breakouts upon request.</footnote>
        <footnote id="F2">The Reporting Person's wife executed a trade order through a broker-dealer which resulted in multiple same-day same-way open market purchases, with the prices ranging from $19.16 to $19.95/share.  The Reporting Person has reported these purchases on an aggregate basis using the weighted average price for the transactions and will provide detailed breakouts upon request.</footnote>
        <footnote id="F3">The Reporting Person disclaims ownership of the securities held by his wife and this report should not be deemed an admission that the Reporting Person is the beneficial owner of his wife's securities for purposes of Section 16 or for any other purposes.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/William J. Armfield, IV by Charles F. McCoy, POA</signatureName>
        <signatureDate>2013-05-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2013-05-02
0
0000100726
UNIFI INC
UFI
0001259388
ARMFIELD WILLIAM J IV
300 N GREENE ST SUITE 2190
GREENSBORO
NC
27401
1
0
0
0
COMMON STOCK
2013-05-02
4
P
0
1326
19.65
A
290699
D
COMMON STOCK
2013-05-02
4
P
0
3500
19.68
A
9639
I
BY WIFE
The Reporting Person executed a trade order through a broker-dealer which resulted in multiple same-day same-way open market purchases, with the prices ranging from $19.05 to $19.84/share.  The Reporting Person has reported these purchases on an aggregate basis using the weighted average price for the transactions and will provide detailed breakouts upon request.
The Reporting Person's wife executed a trade order through a broker-dealer which resulted in multiple same-day same-way open market purchases, with the prices ranging from $19.16 to $19.95/share.  The Reporting Person has reported these purchases on an aggregate basis using the weighted average price for the transactions and will provide detailed breakouts upon request.
The Reporting Person disclaims ownership of the securities held by his wife and this report should not be deemed an admission that the Reporting Person is the beneficial owner of his wife's securities for purposes of Section 16 or for any other purposes.
/s/William J. Armfield, IV by Charles F. McCoy, POA
2013-05-03