As filed with the Securities and Exchange Commission on April 7, 2020

Registration No. 333-            

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

KEROS THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

99 Hayden Avenue, Suite 120, Building E

Lexington, Massachusetts 02421

Tel: (617) 314-6297

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Jasbir Seehra

Chief Executive Officer

Keros Therapeutics, Inc.

99 Hayden Avenue, Suite 120, Building E

Lexington, Massachusetts 02421

Tel: (617) 314-6297

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Approximate date of commencement of proposed sale to the public: As soon as practicable after the effective date of this registration statement.

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box.  ☐

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☒ (333-237212)

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, smaller reporting company or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☐

CALCULATION OF REGISTRATION FEE

This Registration Statement shall become effective upon filing in accordance with Rule 462(b) promulgated under the Securities Act of 1933, as amended.

EXPLANATORY NOTE AND

INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

This Registration Statement (the “Registration Statement”) is being filed with the Securities and Exchange Commission (the “Commission”) with respect to the registration of additional common stock, par value $0.0001 per share (the “Common Stock”), of Keros Therapeutics, Inc. (the “Registrant”), pursuant to Rule 462(b) under the Securities Act of 1933, as amended (the “Securities Act”). This Registration Statement incorporates by reference the contents of, including all amendments and exhibits thereto, the Registration Statement on Form S-1, as amended (File No. 333-237212) (the “Prior Registration Statement”), which the Commission declared effective on April 7, 2020, and is being filed solely for the purpose of registering additional securities of the same class as were included in the Prior Registration Statement.

The Registrant is filing this Registration Statement for the sole purpose of increasing the aggregate offering price of shares to be offered in the public offering by $18,400,000, which includes additional shares that the underwriters have the option to purchase. The additional shares of Common Stock that are being registered for sale are in an amount and at a price that together represent no more than 20% of the maximum aggregate offering price set forth in the Calculation of Registration Fee table contained in the Prior Registration Statement.

The required opinion and consents are listed on the Exhibit Index attached hereto and filed herewith.

EXHIBIT INDEX

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized in Lexington, Massachusetts, on the 7th day of April, 2020.

Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

Exhibit 5.1

Marc A. Recht

T: +1 617 937 2316

mrecht@cooley.com

April 7, 2020

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”), in connection with the filing by the Company of a Registration Statement on Form S-1 (the “Registration Statement”) with the Securities and Exchange Commission pursuant to Rule 462(b) promulgated under the Securities Act of 1933, as amended, covering an underwritten public offering of up to 1,150,000 shares of the Company’s common stock, par value $0.0001 (the “Shares”). The Registration Statement incorporates by reference the registration statement on Form S-1 (No. 333-237212), which was declared effective on April 7, 2020 (the “Prior Registration Statement”), including the prospectus which forms a part of the Prior Registration Statement (the “Prospectus”).

In connection with this opinion, we have examined and relied upon (a) the Registration Statement and the Prospectus, (b) the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, each as currently in effect, (c) the forms of the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws filed as Exhibits 3.3 and 3.4, to the Registration Statement, respectively, each of which is to be in effect upon the closing of the offering contemplated by the Registration Statement 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 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.

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 against payment therefor as described in the Registration Statement and the Prospectus, will be validly issued, fully paid and non-assessable.

Cooley LLP 500 Boylston Street Boston, MA 02116-3736

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

We consent to the reference to our firm under the caption “Legal Matters” in the Prospectus included in the Prior Registration Statement and to the filing of this opinion as an exhibit to the Registration Statement.

Sincerely,

Cooley LLP

Cooley LLP 500 Boylston Street Boston, MA 02116-3736

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

Exhibit 23.1

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We consent to the incorporation by reference in this Registration Statement on Form S-1 filed pursuant to Rule 462(b) of the Securities Act of 1933, as amended, of our report dated February 26, 2020 (April 1, 2020, as to the subsequent events described in Note 15), relating to the consolidated financial statements of Keros Therapeutics, Inc. and its subsidiary included in Registration Statement on Form S-1 (No. 333-237212) of Keros Therapeutics, Inc. We also consent to the reference to us under the heading “Experts” in such Registration Statement.

/s/ Deloitte & Touche LLP

Boston, Massachusetts

April 7, 2020