UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-A

FOR REGISTRATION OF CERTAIN CLASSES OF SECURITIES

PURSUANT TO SECTION 12(b) OR 12(g) OF

THE SECURITIES EXCHANGE ACT OF 1934

RHYTHM PHARMACEUTICALS, INC.

(Exact name of registrant as specified in its charter)

Securities to be registered pursuant to Section 12(b) of the Act:

If this form relates to the registration of a class of securities pursuant to Section 12(b) of the Exchange Act and is effective pursuant to General Instruction A.(c) or (e), check the following box.  x

If this form relates to the registration of a class of securities pursuant to Section 12(g) of the Exchange Act and is effective pursuant to General Instruction A.(d) or (e), check the following box.  o

If this form relates to the registration of a class of securities concurrently with a Regulation A offering, check the following box.  o

Securities Act registration statement or Regulation A offering statement file number to which this form relates:

333-220337 (if applicable)

Securities to be registered pursuant to Section 12(g) of the Act:

None

Item 1. Description of Registrant’s Securities to be Registered.

A description of the common stock, par value $0.001 per share, to be registered hereunder is set forth under the caption “Description of Capital Stock” in the prospectus that constitutes a part of the Registrant’s Registration Statement on Form S-1, File No. 333-220337 (the “Registration Statement”), initially filed with the U.S. Securities and Exchange Commission on September 5, 2017, as subsequently amended by any amendments to such Registration Statement and by any form of prospectus filed pursuant to Rule 424(b) under the Securities Act of 1933, as amended, in connection with such Registration Statement. Such Registration Statement, as amended, and any form of prospectus filed pursuant to Rule 424(b) under the Securities Act of 1933, as amended, that includes such description, are hereby incorporated by reference herein.

Item 2. Exhibits.

Under the Instructions as to Exhibits with respect to Form 8-A, no exhibits are required to be filed because no other securities of the Registrant are registered on The NASDAQ Stock Market LLC and the securities registered hereby are not being registered pursuant to Section 12(g) of the Securities Exchange Act of 1934, as amended.

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SIGNATURE

Pursuant to the requirements of Section 12 of the Securities Exchange Act of 1934, the registrant has duly caused this registration statement to be signed on its behalf by the undersigned, thereto duly authorized.

[Rhythm Pharmaceuticals, Inc. — Signature Page to Form 8-A]

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