The urban agglomeration of Longueuil was created on January 1, 2006, as a result of the de-amalgamation process brought upon by the Charest government. It encompasses all the boroughs that were merged into the previous city of Longueuil and still retains the same area as that mega-city.
The urban agglomeration of Longueuil is coextensive with the territory equivalent to a regional county municipality (TE) and census division (CD) of Longueuil, whose geographical code is 58.
Longueuil (French pronunciation: [lɔ̃ɡœj] ) 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 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 strong de-amalgamation in 2006 (see 2000–2006 municipal reorganization in Quebec).