v3.25.3

Cover - shares	9 Months Ended
Cover - shares	Sep. 30, 2025	Nov. 14, 2025
Document Information [Line Items]
Document Type	10-Q/A
Amendment Flag	false
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2025
Document Fiscal Period Focus	Q3
Document Fiscal Year Focus	2025
Current Fiscal Year End Date	--12-31
Entity File Number	001-42252
Entity Registrant Name	TERRESTRIAL ENERGY INC.
Entity Central Index Key	0002019804
Entity Tax Identification Number	98-1785406
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	2730 W. Tyvola Road
Entity Address, Address Line Two	Suite 100
Entity Address, City or Town	Charlotte
Entity Address, State or Province	NC
Entity Address, Postal Zip Code	28217
City Area Code	646
Local Phone Number	687-8212
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
Elected Not To Use the Extended Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		81,771,423
Common Stock [Member]
Document Information [Line Items]
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Trading Symbol	IMSR
Security Exchange Name	NASDAQ
Redeemable Warrants [Member]
Document Information [Line Items]
Title of 12(b) Security	Redeemable Warrants, each whole warrant exercisable for one Common Stock at a price of $11.50 per share
Trading Symbol	IMSRW
Security Exchange Name	NASDAQ

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Terrestrial Energy Inc. /DE/ 10-Q/A