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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-08-28</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000726517</issuerCik>
        <issuerName>MERCHANTS BANCSHARES INC</issuerName>
        <issuerTradingSymbol>mbvt</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001220453</rptOwnerCik>
            <rptOwnerName>HAVERS THOMAS R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>275 KENNEDY DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SOUTH BURLINGTON</rptOwnerCity>
            <rptOwnerState>VT</rptOwnerState>
            <rptOwnerZipCode>05403</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Senior Vice President</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2006-08-28</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>2500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>23.99</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>23108</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By 401(k)</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>8658.99</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Family Foundation</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>August 1997 Option Grant</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>18.5</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>1999-08-28</value>
            </exerciseDate>
            <expirationDate>
                <value>2007-08-28</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>11655</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>11655</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>August 1998 Option Grant</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>20.33</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2000-08-20</value>
            </exerciseDate>
            <expirationDate>
                <value>2008-08-20</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>10213</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>10213</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>August 1999 Option Grant</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>17.5</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2001-08-19</value>
            </exerciseDate>
            <expirationDate>
                <value>2009-08-19</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>10871</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>10871</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>August 2000 Option Grant</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>15.5</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2002-08-17</value>
            </exerciseDate>
            <expirationDate>
                <value>2010-08-17</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>6816</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6816</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The reporting person's direct ownership includes dividends acquired through a dividend reinvestment plan, which provides for the regular reinvestment of dividends, broad-based participation and operates on substantially the same terms for all plan participants.</footnote>
        <footnote id="F2">These shares were acquired between 1980 and 2005 through the Company's former ESOP plan, which merged with and into the Company's 401(k) plan.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Lisa A. Razo, P.O.A. for Thomas R. Havers</signatureName>
        <signatureDate>2006-08-29</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2006-08-28
0
0000726517
MERCHANTS BANCSHARES INC
mbvt
0001220453
HAVERS THOMAS R
275 KENNEDY DRIVE
SOUTH BURLINGTON
VT
05403
0
1
0
0
Senior Vice President
Common Stock
2006-08-28
4
S
0
2500
23.99
D
23108
I
By 401(k)
Common Stock
8658.99
I
By Family Foundation
August 1997 Option Grant
18.5
1999-08-28
2007-08-28
Common Stock
11655
11655
D
August 1998 Option Grant
20.33
2000-08-20
2008-08-20
Common Stock
10213
10213
D
August 1999 Option Grant
17.5
2001-08-19
2009-08-19
Common Stock
10871
10871
D
August 2000 Option Grant
15.5
2002-08-17
2010-08-17
Common Stock
6816
6816
D
The reporting person's direct ownership includes dividends acquired through a dividend reinvestment plan, which provides for the regular reinvestment of dividends, broad-based participation and operates on substantially the same terms for all plan participants.
These shares were acquired between 1980 and 2005 through the Company's former ESOP plan, which merged with and into the Company's 401(k) plan.
Lisa A. Razo, P.O.A. for Thomas R. Havers
2006-08-29