Lac Saint-Louis is a lake in extreme southwestern Quebec, Canada, adjoining the Island of Montreal at the confluence of the Saint Lawrence and Ottawa Rivers. One can actually see a line in the middle of the lake where the two different-coloured waters meet.

The lake is bounded to the north and east by the Island of Montreal and the west by Île-Perrot. The town of Beauharnois and the Beauharnois power-dam and canal lie to its south. The lake contains a shipping channel, part of the Saint Lawrence Seaway, joining the South Shore and Beauharnois canals.

Several yacht clubs along the shore of Montreal's West Island make this lake a favourite of Montreal boaters.

To the west,at the lock in St. Anne de Bellevue, the lake connects to Lake of Two Mountains another favourite boating venue and home to the Yacht Club in Hudson.