Longueuil is a city in the province of Quebec, Canada. It is the seat of the Montérégie administrative region and the central city of the urban agglomeration of Longueuil. It sits on the south shore of the Saint Lawrence River directly across from Montreal. The population as of the Canada 2016 Census totalled 239,700, making it Montreal's second largest suburb, the fifth most populous city in Quebec and twentieth largest in Canada.