| Document and Entity Information | Mar. 18, 2022 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Mar. 18, 2022 |
| Trading Symbol | JANX |
| Security12b Title | Common Stock, $0.001 par value per share |
| Security Exchange Name | NASDAQ |
| Entity Registrant Name | Janux Therapeutics, Inc. |
| Entity Central Index Key | 0001817713 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| Entity File Number | 001-40475 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 82-2289112 |
| Entity Address, Address Line One | 11099 N. Torrey Pines Road |
| Entity Address, Address Line Two | Suite 290 |
| Entity Address, City or Town | La Jolla |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 92037 |
| City Area Code | 858 |
| Local Phone Number | 750-4700 |
| Entity Information, Former Legal or Registered Name | N/A |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
