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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2021
Mar. 01, 2022
Jun. 30, 2021
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2021    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Entity Registrant Name CULLINAN ONCOLOGY, INC.    
Current Fiscal Year End Date --12-31    
Document Annual Report true    
Document Transition Report false    
Entity Central Index Key 0001789972    
Entity File Number 001-39856    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 81-3879991    
Entity Address, Address Line One One Main Street    
Entity Address, Address Line Two Suite 520    
Entity Address, City or Town Cambridge    
Entity Address, Country MA    
Entity Address, Postal Zip Code 02142    
City Area Code 617    
Local Phone Number 410-4650    
Title of 12(b) Security Common Stock, par value $0.0001 per share    
Trading Symbol CGEM    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Non-accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company true    
Entity Ex Transition Period false    
ICFR Auditor Attestation Flag false    
Entity Shell Company false    
Entity Public Float     $ 717,444,028
Entity Common Stock, Shares Outstanding   44,608,613  
Auditor Name KPMG LLP    
Auditor Location Boston, Massachusetts    
Auditor Firm ID 185    
Documents Incorporated by Reference

DOCUMENTS INCORPORATED BY REFERENCE

The Registrant intends to file a definitive proxy statement pursuant to Regulation 14A in connection with its 2022 Annual Meeting of Stockholders within 120 days of the end of the Registrant’s fiscal year ended December 31, 2021. Portions of such proxy statement are incorporated by reference into Part III of this Annual Report on Form 10-K.