Lake Saint Pierre (French: Lac Saint-Pierre, pronounced [lak sɛ̃ pjɛʁ]; Western Abnaki: Nebesek) 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.
This lake which is 32 kilometres (20 mi) long (excluding Sorel Islands) and 14 kilometres (8.7 mi) wide, is part of the St. Lawrence Seaway. Including its shoreline, islands, and wetlands, the lake is a nature reserve. The body of water is recognized as a Ramsar site and as a Biosphere Reserve, due to the presence of many marshes and wetlands that are frequented by waterfowl. Recreational activities on the river (such as fishing, boating, sailing, swimming, water skiing, nature observation) are active mainly in summer season. Sport fishing is particularly popular, including ice fishing, especially in the great bay of Pointe-du-Lac.