Lac Saint-Pierre is a lake and nature reserve located on the Saint Lawrence River between Sorel and Trois-Rivières east of Montreal. The lake forms part of the Saint Lawrence Seaway. The reserve includes shoreline, islands and wetlands.

This seasonally-flooded area is an important stopping point for hundreds of thousands of migrating waterfowl. It is also an important nesting area for herons. In 1998, it was recognized as a wetland of international significance under the Ramsar Convention.

Lac Saint-Pierre was designated a World Biosphere Reserve by UNESCO in 2000.