The rupture happened around 2:15 a.m. and flooded a stretch of Saint-Michel Boulevard.
The road is closed in both directions between Masson Street and Laurier Avenue.
Const. Dany Richer said the road will likely remain closed for much of the day.
City workers are on the scene to stop the water and begin the cleanup and repair work.
The flood in the Rosemont neighbourhood comes only two days after a water main ruptured in downtown Montreal, flooding a large perimeter and forcing the closure of some buildings on McGill University's campus.