Exhibit 1.1 

RHYTHM PHARMACEUTICALS, INC.

COMMON STOCK

AMENDMENT NO. 1 TO SALES AGREEMENT

February 29, 2024

Cowen and Company, LLC

599 Lexington Avenue

New York, NY 10022

Ladies and Gentlemen:

Reference is made to the Sales Agreement, dated November 2, 2021 (the “Agreement”), by and between Rhythm Pharmaceuticals, Inc., a Delaware corporation (the “Company”), and Cowen and Company, LLC (“Cowen”). The Company and Cowen (collectively, the “Parties”) wish to amend the Agreement, pursuant to Section 15 of the Agreement, to increase the aggregate offering price under the Agreement (this “Amendment”). The Parties therefore hereby agree as follows:

1. On the date hereof, the Company has filed or will file a Prospectus Supplement relating to the offering of up to $200,000,000 of Common Stock (the “February 2024 ATM Prospectus Supplement”), to the base prospectus, dated March 2, 2023, contained in the registration statement on Form S-3 (Registration No. 333-270233). The February 2024 ATM Prospectus Supplement shall update and amend the prospectus supplement filed by the Company on March 2, 2023 relating to the Common Stock offered pursuant to the Agreement in its entirety.

‘The Company agrees that, from time to time during the term of this Agreement, on the terms and subject to the conditions set forth herein, it may issue and sell through Cowen, acting as agent and/or principal, shares (the “Placement Shares”) of the Company’s common stock, par value $0.001 per share (the “Common Stock”), having an aggregate offering price of up to $200,000,000, excluding the aggregate offering price of Placement Shares issued and sold prior to February 29, 2024 pursuant to this Agreement. Notwithstanding anything to the contrary contained herein, the parties hereto agree that compliance with the limitation set forth in this Section 1 on the number of shares of Common Stock issued and sold under this Agreement shall be the sole responsibility of the Company, and Cowen shall have no obligation in connection with such compliance. The issuance and sale of Common Stock through Cowen will be effected pursuant to the Registration Statement (as defined below) filed by the Company and after such Registration Statement has been declared effective by the Securities and Exchange Commission (the “Commission”), although nothing in this Agreement shall be construed as requiring the Company to use the Registration Statement (as defined below) to issue the Common Stock.’

“20. Recognition of the U.S. Special Resolution Regimes.

`(viii) the reasonable fees and disbursements of Cowen's counsel in an amount not to exceed $75,000.' (a) In the event that Cowen is a Covered Entity and becomes subject to a proceeding under a U.S. Special Resolution Regime, the transfer from Cowen of this Agreement, and any interest and obligation in or under this Agreement, will be effective to the same extent as the transfer would be effective under the U.S. Special Resolution Regime if this Agreement, and any such interest and obligation, were governed by the laws of the United States or a state of the United States.

(b) In the event that Cowen is a Covered Entity and Cowen or a BHC Act Affiliate of Cowen becomes subject to a proceeding under a U.S. Special Resolution Regime, Default Rights under this Agreement that may be exercised against Cowen are permitted to be exercised to no greater extent than such Default Rights could be exercised under the U.S. Special Resolution Regime if this Agreement were governed by the laws of the United States or a state of the United States.

(c) For purposes of this Section 20; (a) “BHC Act Affiliate” has the meaning assigned to the term “affiliate” in, and shall be interpreted in accordance with, 12 U.S.C. § 1841(k), (b) “Covered Entity” means any of the following: (i) a “covered entity” as that term is defined in, and interpreted in accordance with, 12 C.F.R. § 252.82(b); (ii) a “covered bank” as that term is defined in, and interpreted in accordance with, 12 C.F.R. § 47.3(b); or (iii) a “covered FSI” as that term is defined in, and interpreted in accordance with, 12 C.F.R. § 382.2(b), (c) “Default Right” has the meaning assigned to that term in, and shall be interpreted in accordance with, 12 C.F.R. §§ 252.81, 47.2 or 382.1, as applicable, and (d) “U.S. Special Resolution Regime” means each of (i) the Federal Deposit Insurance Act and the regulations promulgated thereunder and (ii) Title II of the Dodd-Frank Wall Street Reform and Consumer Protection Act and the regulations promulgated thereunder.”

6. For the avoidance of doubt, any reference to “Prospectus” in the Agreement shall be deemed to include the February 2024 ATM Prospectus Supplement.

7. Governing Law. THIS AMENDMENT AND ANY CLAIM, CONTROVERSY OR DISPUTE ARISING UNDER OR RELATED TO THIS AMENDMENT SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK, WITHOUT REGARD TO ITS CHOICE OF LAW PROVISIONS.

8. Counterparts. This Amendment may be executed in any number of counterparts, each of which shall be deemed to be an original, but all such counterparts shall together constitute one and the same agreement. Counterparts may be delivered via facsimile, electronic mail (including any electronic signature covered by the U.S. federal ESIGN Act of 2000, Uniform Electronic Transactions Act, the Electronic Signatures and Records Act or other applicable law, e.g., www.docusign.com) or other transmission method and any counterpart so delivered shall be deemed to have been duly and validly delivered and be valid and effective for all purposes.

9. Agreement Remains in Effect. Except as provided herein, all provisions, terms and conditions of the Agreement shall remain in full force and effect. As amended hereby, the Agreement is ratified and confirmed in all respects.

Terms used herein but not otherwise defined are used herein as defined in the Agreement.

(Signature page follows)

If the foregoing correctly sets forth the understanding between the Company and Cowen, please so indicate in the space provided below for that purpose, whereupon this letter shall constitute a binding agreement between the Company and Cowen.

Exhibit 5.1

	200 Clarendon Street
	Boston, Massachusetts 02116
	Tel: +1.617.948.6000 Fax: +1.617.948.6001
	www.lw.com
	FIRM / AFFILIATE OFFICES
	Austin	Milan
	Beijing	Munich
	Boston	New York
	Brussels	Orange County
	Century City	Paris
	Chicago	Riyadh
February 29, 2024	Dubai	San Diego
	Düsseldorf	San Francisco
	Frankfurt	Seoul
	Hamburg	Silicon Valley
	Hong Kong	Singapore
	Houston	Tel Aviv
	London	Tokyo
	Los Angeles	Washington, D.C.
Rhythm Pharmaceuticals, Inc.	Madrid

222 Berkeley Street

12th Floor

Boston, Massachusetts 02116

To the addressee set forth above:

We have acted as special counsel to Rhythm Pharmaceuticals, Inc., a Delaware corporation (the “Company”), in connection with the sale through Cowen and Company, LLC, as sales agent (the “Sales Agent”), from time to time by the Company of shares of common stock, $0.001 par value per share, having an aggregate offering price of up to $200.0 million (the “Shares”) pursuant to (i) a registration statement on Form S-3 under the Securities Act of 1933, as amended (the “Act”), filed with the Securities and Exchange Commission (the “Commission”) on March 2, 2023 (Registration No. 333-270233) (the “Registration Statement”), (ii) a base prospectus dated March 2, 2023 (the “Base Prospectus”), (iii) a related prospectus supplement filed with the Commission on the date hereof pursuant to Rule 424(b) under the Act (together with the Base Prospectus, the “Prospectus”), and (iv) that certain Sales Agreement, dated November 2, 2021, by and between the Company and the Sales Agent, as amended by Amendment No. 1 to the Sales Agreement, dated as of the date hereof (the “Sales Agreement”). This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement or related Prospectus, other than as expressly stated herein with respect to the issue of the Shares.

As such counsel, we have examined such matters of fact and questions of law as we have considered appropriate for purposes of this letter. With your consent, we have relied upon certificates and other assurances of officers of the Company and others as to factual matters without having independently verified such factual matters. We are opining herein as to the General Corporation Law of the State of Delaware (the “DGCL”), and we express no opinion with respect to any other laws.

February 29, 2024

Page 2

Subject to the foregoing and the other matters set forth herein, it is our opinion that, upon the completion of all Corporate Proceedings (as defined below) relating to the Shares, when the Shares shall have been duly registered on the books of the transfer agent and registrar therefor in the name or on behalf of the purchasers, upon issuance, delivery and payment therefor in an amount not less than the par value thereof in accordance with the Corporate Proceedings and the terms of the Sales Agreement, the Shares to be issued and sold by the Company pursuant to the Sales Agreement will be duly authorized by all necessary corporate action of the Company and such Shares will be validly issued, fully paid and nonassessable. In rendering the foregoing opinion, we have assumed that (i) the Company will comply with all applicable notice requirements regarding uncertificated shares provided in the DGCL, (ii) upon the issuance of any of the Shares, the total number of shares of Common Stock issued and outstanding will not exceed the total number of shares of Common Stock that the Company is then authorized to issue under its certificate of incorporation, and (iii) certain terms of the Shares to be issued by the Company from time to time will be authorized and approved by the board of directors of the Company or one or more committees thereof established by the board of directors of the Company with the authority to issue and sell Shares pursuant to the Sales Agreement in accordance with the DGCL, the Company’s certificate of incorporation, the by-laws of the Company and certain resolutions of the board of directors of the Company and one or more committees thereof (with such approvals referred to herein as the “Corporate Proceedings”) prior to issuance thereof.

This opinion is for your benefit in connection with the Registration Statement and may be relied upon by you and by persons entitled to rely upon it pursuant to the applicable provisions of the Act. We consent to your filing this opinion as an exhibit to the Company’s Form 8-K dated February 29, 2024 and to the reference to our firm contained in the Registration Statement and in the Prospectus under the heading “Legal Matters.” In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission thereunder.