About 1,200 military personnel have been deployed to support emergency services responding to the flooding.
In a statement announcing the 48-hour state of emergency, the city of Montreal said forecasts suggested that the situation would not be short-lived.
Environment Canada was warning that water levels in the Rivière des Prairies were expected to rise by 20 centimeters overnight, weakening dikes, breakwaters and dams set up in key areas to prevent water infiltration, the statement said.