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 2021 Census totalled 254,483, making it Montreal's second largest suburb, the fifth most populous city in Quebec and twentieth largest in Canada.

Charles le Moyne de Longueuil et de Châteauguay, was a French officer and merchant who was a prominent figure in the early days of Montreal. Born in Dieppe, France in Normandy, he came to New France in 1641. He became lord of Longueuil in Canada.