Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 08, 2019
Document And Entity Information [Abstract]
Trading Symbol	SESN
Entity Registrant Name	Sesen Bio, Inc.
Entity Central Index Key	0001485003
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Emerging Growth Company	true
Entity Small Business	true
Entity Transition Period Flag	true
Entity Common Stock, Shares Outstanding		77,464,781