Longueuil is a city located in the province of Quebec, Canada. It serves as the seat of the Montérégie administrative region and functions as the central city of the urban agglomeration of Longueuil.
Situated on the south shore of the Saint Lawrence River, the city sits directly across from Montreal. As of the 2021 Census, Longueuil had a population of 254,483, making it the second largest suburb of Montreal, the fifth most populous city in Quebec, and the twentieth largest city in Canada.