Longueuil ( /lɒŋˈɡɔɪ/; ) 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 2011 Census totaled 231,409, making it the third largest city in Greater Montreal, the fifth most populous city in Quebec and nineteenth largest in Canada.
Charles Le Moyne founded Longueuil as a seigneurie in 1657. It would become a parish in 1845, a village in 1848, a town in 1874 and a city in 1920. Between 1961 and 2002, Longueuil's borders grew three times, as it was amalgamated with surrounding municipalities; there was a partial de-amalgamation in 2006 (see 2000–2006 municipal reorganization in Quebec).
Longueuil is a residential, commercial and industrial city. It incorporates some urban features, but is essentially a suburb. Longueuil can be classified as a commuter town as a large portion of its residents commute to work in Montreal. Most buildings are single-family homes constructed in the post-war period. The city consists of three boroughs: Le Vieux-Longueuil, Saint-Hubert and Greenfield Park.
Longueuil is the seat of the judicial district of Longueuil. Residents of the city are called Longueuillois.