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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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<SEC-DOCUMENT>0001413306-10-000001.txt : 20100330
<SEC-HEADER>0001413306-10-000001.hdr.sgml : 20100330
<ACCEPTANCE-DATETIME>20100330173031
ACCESSION NUMBER:		0001413306-10-000001
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20100329
FILED AS OF DATE:		20100330
DATE AS OF CHANGE:		20100330

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Grais Linda S
		CENTRAL INDEX KEY:			0001413306

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-22873
		FILM NUMBER:		10714926

	MAIL ADDRESS:	
		STREET 1:		C/O INTERWEST PARTNERS
		STREET 2:		2710 SAND HILL ROAD SECOND FLOOR
		CITY:			MENLO PARK
		STATE:			CA
		ZIP:			94025

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			ARCA biopharma, Inc.
		CENTRAL INDEX KEY:			0000907654
		STANDARD INDUSTRIAL CLASSIFICATION:	IN VITRO & IN VIVO DIAGNOSTIC SUBSTANCES [2835]
		IRS NUMBER:				363855489
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		8001 ARISTA PLACE
		STREET 2:		SUITE 200
		CITY:			BROOMFIELD
		STATE:			CO
		ZIP:			80021
		BUSINESS PHONE:		720-940-2200

	MAIL ADDRESS:	
		STREET 1:		8001 ARISTA PLACE
		STREET 2:		SUITE 200
		CITY:			BROOMFIELD
		STATE:			CO
		ZIP:			80021

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	NUVELO INC
		DATE OF NAME CHANGE:	20030203

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	HYSEQ INC
		DATE OF NAME CHANGE:	19970610
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>primary_doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-03-29</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000907654</issuerCik>
        <issuerName>ARCA biopharma, Inc.</issuerName>
        <issuerTradingSymbol>ABIO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001413306</rptOwnerCik>
            <rptOwnerName>Grais Linda S</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O INTERWEST PARTNERS</rptOwnerStreet1>
            <rptOwnerStreet2>2710 SAND HILL ROAD SECOND FLOOR</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</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>2010-03-29</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>23600</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>7.98</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>679098</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Partnership</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-03-29</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>187209</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>7.3</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>491889</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>by Partnership</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The shares were sold at prices between $7.90 and $8.21 per share, resulting in an average price of $7.98 per share. Upon request, the reporting person will provide to the SEC, the issuer or securtiy holder of the issuer, full information regarding the number of shares sold at each price.</footnote>
        <footnote id="F2">The shares were sold at prices between $6.90 and $7.89 per share, resulting in an average price of $7.30 per share. Upon request, the Reporting Person will provide to the SEC, the issuer or security holder of the issuer, full information regarding the number of shares sold at each price.</footnote>
        <footnote id="F3">These shares are owned directly by InterWest Partners IX, LP. Ms. Grais, one of ARCA's directors, is a venture member of InterWest Management Partners IX, LLC, the general partner of InterWest Partners IX, LP. Ms. Grais has shared voting and dispositive powers over the shares held by InterWest Partners IX, LP, and disclaims beneficial ownership of such shares except to the extent of her pro rata partnership interest therein.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Linda S. Grais</signatureName>
        <signatureDate>2010-03-30</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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