Châteauguay is an off-island suburb of Montreal, in southwestern Quebec, located on both the Châteauguay River and Lac St-Louis, which is a …Châteauguay is an off-island suburb of Montreal, in southwestern Quebec, located on both the Châteauguay River and Lac St-Louis, which is a section of the St. Lawrence River. The population of the city of Châteauguay at the 2021 Census was 50,815, and the population centre was 75,891.