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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2020-05-14</periodOfReport>

    <dateOfOriginalSubmission>2020-05-14</dateOfOriginalSubmission>

    <issuer>
        <issuerCik>0001174850</issuerCik>
        <issuerName>NICOLET BANKSHARES INC</issuerName>
        <issuerTradingSymbol>NCBS</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001243734</rptOwnerCik>
            <rptOwnerName>WEYERS ROBERT J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O NICOLET NATIONAL BANK</rptOwnerStreet1>
            <rptOwnerStreet2>111 NORTH WASHINGTON STREET</rptOwnerStreet2>
            <rptOwnerCity>GREEN BAY</rptOwnerCity>
            <rptOwnerState>WI</rptOwnerState>
            <rptOwnerZipCode>54301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-05-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1930</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>46.6101</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>25250</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Weyers Family Limited Partnership</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>82971</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">This report on Form 4/A amends the Form 4 filed by the reporting person on May 14, 2020.  This report on Form 4/A is being filed to increase the number of shares indirectly owned by the Weyers Family Limited Partnership, for shares acquired on May 14, 2020.</footnote>
        <footnote id="F2">Represents the holdings of Weyers Family Limited Partnership, of which the reporting person is a general partner.  The reporting person disclaims beneficial ownership of registrant common stock held by Weyers Family Limited Partnership except to the extent of his pecuniary interest.</footnote>
        <footnote id="F3">This report on Form 4/A amends the Form 4 filed by the reporting person on May 14, 2020.  This report on Form 4/A is being filed to show the correct number of direct shares beneficially owned which eliminates the shares purportedly acquired on May 13, 2020 and May 14, 2020.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Ann K. Lawson, as attorney-in-fact for Robert J. Weyers</signatureName>
        <signatureDate>2020-12-18</signatureDate>
    </ownerSignature>
</ownershipDocument>
