Cover - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Mar. 08, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-36296
Entity Registrant Name	Sesen Bio, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-2025616
Entity Address, Address Line One	245 First Street
Entity Address, Address Line Two	Suite 1800
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	444-8550
Title of 12(b) Security	Common Stock, $0.001 par value
Trading Symbol	SESN
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 Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
ICFR Auditor Attestation Flag	false
Entity Public Float			$ 84,000
Entity Common Stock, Shares Outstanding		167,578,637
Entity Central Index Key	0001485003
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Filer Category	Non-accelerated Filer