| Document and Entity Information | Mar. 10, 2022 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001708493 |
| Document Type | 8-K |
| Document Period End Date | Mar. 10, 2022 |
| Entity Registrant Name | Harpoon Therapeutics, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-38800 |
| Entity Tax Identification Number | 47-3458693 |
| Entity Address, Address Line One | 131 Oyster Point Blvd |
| Entity Address, Address Line Two | Suite 300 |
| Entity Address, City or Town | South San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94080 |
| City Area Code | 650 |
| Local Phone Number | 443-7400 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, par value $0.0001 per share |
| Trading Symbol | HARP |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
