Cover Page - shares	6 Months Ended
	Jun. 30, 2020	Aug. 03, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2020
Document Transition Report	false
Entity File Number	001-38199
Entity Registrant Name	Tremont Mortgage Trust
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	82-1719041
Entity Address, Address Line One	Two Newton Place
Entity Address, Address Line Two	255 Washington Street
Entity Address, Address Line Three	Suite 300
Entity Address, City or Town	Newton
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02458-1634
City Area Code	617
Local Phone Number	796-8317
Title of 12(b) Security	Common Shares of Beneficial Interest
Trading Symbol	TRMT
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Smaller Business Entity	true
Emerging Growth Company	true
Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		8,253,836
Entity Central Index Key	0001708405
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2