Fort Saint-Jean, Fort and national historic site in Saint-Jean-sur-Richelieu, Canada.

Fort Saint-Jean is a fort on the Richelieu River whose second incarnation, built in 1748, measured 200 by 200 feet and had four bastions, two of them masonry. Its stone bastions survived the 1760 fire and were reused by British troops. The site now houses the Royal Military College Saint-Jean, and buildings erected by the Royal Engineers in 1839 still stand.

The Carignan-Salières Regiment built the first Fort Saint-Jean in 1666 as a small square palisade fort with four bastions, guarding the portage above the Chambly rapids. A second fort, 200 feet square with two stone bastions, replaced it in 1748 and was burned by its French garrison in 1760. The British built a third fort in 1775; after the 1837–1838 Patriot Revolt, Royal Engineers added buildings in 1839 that still stand.

The site served as a military fortress for generations before becoming the Royal Military College Saint-Jean in 1952, marking a shift from defense to education. This transformation made it one of Canada's earliest bilingual military training centers and shaped how locals view the location today.

The location sits roughly 40 kilometers southeast of Montreal and is accessible by car from the city. Visitors can walk the grounds freely to view the outdoor structures and indoor exhibits displaying military objects and historical information.

The 45-day siege delayed Montgomery's march so much that he reached Quebec City in early December instead of mid-October; only about 300 of his roughly 2,000 men got there. The fort's stand thus helped doom the American assault of 31 December 1775.