|Location||Montérégie region, southwestern Quebec|
|Primary inflows||Beauharnois Canal, St. Lawrence River, Ottawa River, Saint-Charles River|
|Primary outflows||St. Lawrence River|
|Surface elevation||21 m (69 ft)|
Lake St. Louis is a widening of the St. Lawrence River in the Hochelaga Archipelago it is also fed by the Ottawa River via the lock in Sainte-Anne-de-Bellevue, the Beauharnois Canal, the Soulanges Canal, the St. Louis River, and the Châteauguay River.
The lake is bounded to the north and east by the Island of Montreal, by Beauharnois-Salaberry, Roussillon, and Vaudreuil-Soulanges. The town of Beauharnois with its power-dam and canal lie to the south.
The West Island shore is mostly built-up with private houses, but includes some parks and clubs such as the Pointe-Claire Canoe Club, and the Pointe-Claire Yacht Club. Islands in the lake include l'Île-Dorval, and Dowker Island. Lake St. Louis is the second of three fluvial lakes on the St. Lawrence River. Upstream of it is Lake St. Francis, and downstream is Lake Saint-Pierre. Its average flow is 8,400 cubic metres per second (300,000 cu ft/s).
Many species of fish are present in the lake, including yellow perch.
A small map by Samuel Champlain of 1611 names the lake. 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».
- Environment Canada - fluvial lakes of the St. Lawrence
- "Lac Saint-Louis". Commission de toponymie. Retrieved 25 June 2014.
- "Beaconsfield residents had no idea backyard creek was polluted with fecal coliform". www.cbc.ca. Retrieved 2016-04-09.
- "Pointe-Claire pays $435,000 cleanup bill at PCB site on Hymus". Montreal Gazette. Retrieved 2016-04-09.