Exhibit 5.1

July 2, 2020

ARCA biopharma, Inc.
11080 CirclePoint Road, Suite 140
Westminster, CO 80020

Ladies and Gentlemen:

You have requested our opinion, as counsel to ARCA biopharma, Inc., a Delaware corporation (the “Company”), with respect to certain matters in connection with the offering by the Company of up to $7,636,600 of the Company’s common stock, par value $0.001 (the “Shares”), pursuant to a Registration Statement on Form S-3 (No. 333-238067) (the “Registration Statement”), filed with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Act”), the prospectus included in the Registration Statement (the “Base Prospectus”), and the prospectus supplement dated July 2, 2020, filed with the Commission pursuant to Rule 424(b) under the Act (together with the Base Prospectus, the “Prospectus”) The Shares are to be sold by the Company in accordance with a Capital on Demand™ Sales Agreement, dated January 11, 2017 and as amended on August 21, 2017, January 25, 2019, March 11, 2019, May 9, 2019, May 20, 2019, June 28, 2019 and July 2, 2020, between the Company and JonesTrading Institutional Services LLC (the “Agreement”), as described in the Prospectus.

In connection with this opinion, we have examined and relied upon the Registration Statement and the Prospectus, the Agreement, the Company’s Amended and Restated Certificate of Incorporation, as amended, and Second Amended and Restated Bylaws, as amended, and originals or copies certified to our satisfaction of such records, documents, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below. In rendering this opinion, we have assumed the genuineness of all signatures; the authenticity of all documents submitted to us as originals; the conformity to originals of all documents submitted to us as copies; the accuracy, completeness and authenticity of certificates of public officials; and the due authorization, execution and delivery of all documents by all persons other than the Company where authorization, execution and delivery are prerequisites to the effectiveness thereof.  As to certain factual matters, we have relied upon a certificate of an officer of the Company and have not independently verified such matters.  

We have assumed (i) that each sale of Shares will be duly authorized by the Board of Directors of the Company, a duly authorized committee thereof or a person or body pursuant to an authorization granted in accordance with Section 152 of the General Corporation Law of the State of Delaware (the “DGCL”), (ii) that no more than 7,636,600 Shares will be sold under the Agreement and (iii) that the price at which the Shares are sold will equal or exceed the par value of the Shares.  We express no opinion to the extent that future issuances of securities of the Company and/or anti-dilution adjustments to outstanding securities of the Company cause the number of shares of the Company’s common stock outstanding or issuable upon conversion or exercise of outstanding securities of the Company to exceed the number of Shares then issuable under the Agreement.

Our opinion herein is expressed solely with respect to the DGCL.  Our opinion is based on these laws as in effect on the date hereof.  We express no opinion to the extent that any other laws are applicable to the subject matter hereof and express no opinion and provide no assurance as to compliance with any federal or state securities law, rule or regulation.

Cooley LLP   380 Interlocken Crescent   Suite 900   Broomfield, CO   80021-8023
t: (720) 566-4000  f: (720) 566-4099  cooley.com

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares, when sold and issued against payment therefor in accordance with the Agreement, the Registration Statement and the Prospectus, will be validly issued, fully paid and non-assessable.

We consent to the reference to our firm under the caption “Legal Matters” in the Prospectus and to the filing of this opinion as an exhibit to the Company’s Current Report on Form 8-K to be filed with the Commission for incorporation by reference into the Registration Statement.

Very truly yours,

Cooley LLP

Cooley LLP   380 Interlocken Crescent   Suite 900   Broomfield, CO   80021-8023
t: (720) 566-4000  f: (720) 566-4099  cooley.com