Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Jul. 31, 2019
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Trading Symbol HARP  
Entity Registrant Name Harpoon Therapeutics, Inc.  
Entity Central Index Key 0001708493  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Common Stock, Shares Outstanding   24,564,479
Entity Shell Company false  
Entity Tax Identification Number 473458693  
Entity File Number 001-38800  
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  
Entity Current Reporting Status Yes