Cornwall Island is an island in the Saint Lawrence River, directly south of the city of Cornwall. The island is located completely within Canada, but it is also part of the Akwesasne Mohawk reserve, which straddles the Canada–United States border and the Quebec–Ontario border. The Seaway International Bridge, with a channel crossing on each side of the island, provides road access to Cornwall Island from both Canada and the United States.

The St. Lawrence River is a large international river in the middle latitudes of North America connecting the Great Lakes to the North Atlantic Ocean. Its waters flow in a northeasterly direction from Lake Ontario to the Gulf of St. Lawrence, traversing Ontario and Quebec in Canada and New York in the United States. A section of the river demarcates the Canada–U.S. border.