Lake Saint Pierre is a lake in Quebec, Canada, a widening of the Saint Lawrence River between Sorel-Tracy and Trois-Rivières. It is located downstream, and northeast, of Montreal; and upstream, and southwest, of Quebec City. The end of the lake delimits the beginning of the estuary of Saint Lawrence.

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.