| Cover | Dec. 31, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K/A |
| Amendment Flag | false |
| Document Period End Date | Dec. 31, 2020 |
| Entity File Number | 001-38223 |
| Entity Registrant Name | RHYTHM PHARMACEUTICALS, INC. |
| Entity Central Index Key | 0001649904 |
| Entity Tax Identification Number | 46-2159271 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 222 Berkeley Street |
| Entity Address, Address Line Two | 12th Floor |
| Entity Address, City or Town | Boston |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02116 |
| City Area Code | 857 |
| Local Phone Number | 264-4280 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.001 par value per share |
| Trading Symbol | RYTM |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Elected Not To Use the Extended Transition Period | true |
