Fort Longueuil, Archaeological site and national historic fort in Le Vieux-Longueuil, Canada.

Fort Longueuil was a stone fort with four towers built between 1685 and 1690 as the fortified residence of Charles Le Moyne de Longueuil. It measured 68 by 46 metres and held a house, a chapel, a guard house, barns and stables. Demolished in 1810, its remains lie beneath the Saint-Antoine-de-Padoue Cathedral and nearby streets.

Charles Le Moyne de Longueuil built the stone fort with four towers between 1685 and 1690 as his fortified residence on the Seigneury of Longueuil. In 1700, Louis XIV erected the seigneury into the Barony of Longueuil, making Charles the only Canadian-born person raised to that rank by a French king. The fort was later occupied by American troops during the American Revolutionary War and then by the British, before being demolished in 1810 due to poor condition.

The fort's name survives in the toponymy of Old Longueuil, and its footprint underlies Saint-Charles Street, Chambly Road and the Co-Cathedral of Saint-Antoine-de-Padoue. Excavations in 1971 uncovered the north-east wall and the east tower beneath Chambly Road; these remains were moved a few metres onto the cathedral grounds, where visitors can see them.

The archaeological remains can be viewed through a window in the floor of a Royal Bank of Canada branch located in the downtown area. This viewing point is easily accessible during regular business hours for those passing through the city center.

The fort's footprint is split among several modern buildings: the Co-Cathedral of Saint-Antoine-de-Padoue, Saint-Charles Street, Chambly Road, a bank branch and a restaurant. Excavations in 1971 uncovered the north-east wall and east tower beneath Chambly Road; the remains were moved a few metres to the cathedral grounds.