Lake Saint-Louis (French: Lac Saint-Louis, pronounced [lak sɛ̃ lwi]) is a lake in southwestern Quebec, Canada, at the confluence of the Saint Lawrence and Ottawa rivers.

Lake St. Louis is a widening of the St. Lawrence River in the Hochelaga Archipelago.

The lake is bounded to the north and the east by the Island of Montreal, by Beauharnois-Salaberry, Roussillon, and Vaudreuil-Soulanges.

The same year, Champlain reported that a young man named Louys was drowned in what is now known as the Lachine Rapids, and in 1870 Charles-Honoré Laverdière stated that the rapids, and later the lake, were named in honour of the drowned man.

A 1656 Jesuit account describes a crossing «Lac Saint Louys».