EX-5.1

Exhibit 5.1

LOGO

Marc A. Recht

T: +1 617 937 2316

mrecht@cooley.com

April 17, 2020

Keros Therapeutics, Inc.

99 Hayden Avenue

Bldg. E, Suite 120

Lexington, MA 02421

Ladies and Gentlemen:

We have represented Keros Therapeutics, Inc., a Delaware corporation (the “Company”), in connection with the filing by the Company of a Registration Statement on Form S-8 (the “Registration Statement”) with the Securities and Exchange Commission covering the offering of up to 3,290,172 shares (the “Shares”) of the Company’s Common Stock, par value $0.0001 per share (the “Common Stock”), consisting of (i) 1,116,301 shares of Common Stock issuable pursuant to the Company’s 2017 Equity Incentive Plan, as amended (the “2017 Plan”), (ii) 1,991,530 shares of Common Stock issuable pursuant to the Company’s 2020 Equity Incentive Plan (the “2020 Plan”), and (iii) 182,341 shares of Common Stock issuable pursuant to the Company’s 2020 Employee Stock Purchase Plan (together with the 2017 Plan and the 2020 Plan, the “Plans”).

In connection with this opinion, we have examined and relied upon (a) the Registration Statement and the related prospectuses, (b) the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, each as currently in effect, (c) the Plans, and (d) 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. We have assumed the genuineness and authenticity of all documents submitted to us as originals, the conformity to originals of all documents submitted to us as copies thereof, the accuracy, completeness and authenticity of certificates of public officials, and the due execution and delivery of all documents by all persons other than the Company where execution and delivery are a prerequisite 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.

Our opinion is expressed only with respect to the General Corporation Law of the State of Delaware. 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, we are of the opinion that the Shares, when sold and issued in accordance with the Plans, the Registration Statement and the related prospectuses, will be validly issued, fully paid and nonassessable (except as to shares issued pursuant to certain deferred payment arrangements, which will be fully paid and nonassessable when such deferred payments are made in full).

Cooley LLP 500 Boylston Street Boston, MA 02116-3736

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