Fort Saint-Jean is a fort in the Canadian province of Quebec located on the Richelieu River. The fort was first built in 1666 by soldiers of the Carignan-Salières Regiment of France who had travelled to New France to assist the young colony. It was part of a series of forts built along the Richelieu River. Over the years, it was destroyed and rebuilt several times, but it is, after Quebec City, the military site that has been occupied non-stop for the longest time in Canada. The fort is designated as a National Historic Site of Canada, and it currently houses the Royal Military College Saint-Jean. The fort has been continually occupied since 1748, and is the core from which the city of Saint-Jean-sur-Richelieu, Quebec grew around. Fort Saint-Jean played a crucial role in the British defense strategy during the 1775 American invasion of the Province of Quebec.

The Richelieu River is a river of Quebec, Canada, and a major right-bank tributary of the St. Lawrence River. It rises at Lake Champlain, from which it flows northward through Quebec and empties into the St. Lawrence. It was formerly known by the French as the Iroquois River and the Chambly River, and was named for Cardinal Richelieu, the powerful minister under Louis XIII.