www.investorvote.com/RYTM Step 1: Go to www.investorvote.com/RYTM. Step 2: Click on the icon on the right to view meeting materials. Step 3: Return to the investorvote.com window and follow the instructions on the screen to log in. Online - Before the Meeting Go to www.investorvote.com/RYTM or scan the QR code — login details are located in the shaded bar below. Stockholder Meeting Notice 03SZNB + + Important Notice Regarding the Availability of Proxy Materials for the Rhythm Pharmaceuticals, Inc. Stockholder Meeting to be Held on June 21, 2023 Under Securities and Exchange Commission rules, you are receiving this notice that the proxy materials for the annual stockholders’ meeting are available on the Internet. Follow the instructions below to view the materials and vote online or request a copy. The items to be voted on and location of the annual meeting are on the reverse side. Your vote is important! This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. We encourage you to access and review all of the important information contained in the proxy materials before voting. The 2023 Notice and Proxy Statement and 2022 Annual Report are available at: Obtaining a Copy of the Proxy Materials – If you want to receive a copy of the proxy materials, you must request one. There is no charge to you for requesting a copy. Please make your request as instructed on the reverse side on or before June 8, 2023 to facilitate timely delivery. 2NOT Easy Online Access — View your proxy materials and vote. When you go online, you can also help the environment by consenting to receive electronic delivery of future materials. Online - During the Meeting Go to www.meetnow.global/MMAD95R. To access the virtual meeting, you must have the information that is printed in the shaded bar located below. Step 4: Make your selections as instructed on each screen for your delivery preferences. Step 5: Vote your shares.

Here’s how to order a copy of the proxy materials and select delivery preferences: Current and future delivery requests can be submitted using the options below. If you request an email copy, you will receive an email with a link to the current meeting materials. PLEASE NOTE: You must use the number in the shaded bar on the reverse side when requesting a copy of the proxy materials. — Internet – Go to www.investorvote.com/RYTM. — Phone – Call us free of charge at 1-866-641-4276. — Email – Send an email to investorvote@computershare.com with “Proxy Materials Rhythm Pharmaceuticals, Inc.” in the subject line. Include your full name and address, plus the number located in the shaded bar on the reverse side, and state that you want a paper copy of the meeting materials. To facilitate timely delivery, requests for a paper copy of proxy materials must be received by June 8, 2023. Rhythm Pharmaceuticals, Inc.’s Annual Meeting of Stockholders will be held on June 21, 2023, at 2:00 PM Eastern Time. The Annual Meeting will be a completely virtual meeting, which will be conducted via live webcast. To access the virtual meeting, you must have the information that is printed in the shaded bar located on the reverse side of this form. Proposals to be voted on at the meeting are listed below along with the Board of Directors’ recommendations. The Board of Directors recommend a vote FOR all the nominees listed and FOR Proposal 2 and 3: 1. Election of the Class III directors, each to serve for a three year term until the 2026 Annual Meeting of Stockholders and until their respective successors are duly elected and qualified. Nominees 01 - Camille L. Bedrosian, M.D. 02 - David W.J. McGirr 03 - David P. Meeker, M.D. 2. Ratification of the appointment of Ernst & Young LLP as the independent registered public accounting firm for the Company for the fiscal year ending December 31, 2023. 3. Approval, on an advisory (non-binding) basis, of the compensation of the named executive officers of the Company. PLEASE NOTE – YOU CANNOT VOTE BY RETURNING THIS NOTICE. To vote your shares you must go online or request a paper copy of the proxy materials to receive a proxy card. Stockholder Meeting Notice