| Cover | May 11, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | May 11, 2021 |
| Entity Registrant Name | PRAXIS PRECISION MEDICINES, INC. |
| Amendment Flag | false |
| Entity Central Index Key | 0001689548 |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-39620 |
| Entity Tax Identification Number | 47-5195942 |
| Entity Address, Address Line One | One Broadway |
| Entity Address, Address Line Two | 16th Floor |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02142 |
| City Area Code | 617 |
| Local Phone Number | 300-8460 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.0001 par value per share |
| Trading Symbol | PRAX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
