Exhibit 5.1

Marc A. Recht

+1 617 937 2316

mrecht@cooley.com

December 12, 2022

Keros Therapeutics, Inc.

99 Hayden Avenue

Bldg. E, Suite 120

Lexington, MA 02421

Ladies and Gentlemen:

We have acted as counsel to Keros Therapeutics, Inc., a Delaware corporation (the “Company”), with respect to certain matters in connection with the offering by the Company of shares of the Company’s common stock, par value $0.0001 (the “Common Stock”), having aggregate sale proceeds of up to $250,000,000 (the “Shares”), a Registration Statement on Form S-3 (Registration Statement No. 333‑255724) (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 relating to the Shares dated December 12, 2022 and 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 the Sales Agreement, dated May 3, 2021, by and among SVB Securities LLC and the Company (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 Company’s certificate of incorporation and bylaws, each as currently in effect, the Agreement and originals, or copies certified to our satisfaction, of such other 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 of such documents. 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 25,000,000 Shares will be sold pursuant to the Agreement and (iii) that the price at which the Shares are sold will equal or exceed the par value of the Common Stock. 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 and/or other matters cause the number of shares of Common Stock issuable under the Agreement to exceed the number of shares of common stock the available for issuance under the Company’s certificate of incorporation.

Our opinion herein is expressed solely with respect to the DGCL. 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.

On the basis of the foregoing, and in reliance thereon, and subject to the qualifications herein stated, we are of the opinion that the Shares, when issued against payment therefor in accordance with the

Cooley LLP 500 Boylston Street Boston, MA 02116-3736

Phone: (617) 937-2300 Fax: (617) 937-2400

Keros Therapeutics, Inc.

December 12, 2022

Page Two

Agreement, the Registration Statement and the Prospectus, will be validly issued, fully paid and nonassessable.

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 a Current Report on Form 8-K to be filed with the Commission for incorporation by reference into the Registration Statement.

This opinion is expressed as of the date hereof, and we disclaim any responsibility to advise you of any changes in the facts stated or assumed herein or of any changes in applicable law.

Sincerely,

Cooley LLP

By: /s/ Marc A. Recht    

Marc A. Recht

Cooley LLP 500 Boylston Street Boston, MA 02116-3736

Phone: (617) 937-2300 Fax: (617) 937-2400