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.
At least 180 homes had been evacuated, the city said, and the Red Cross was providing shelter to those residents forced out.
"Montreal is in the middle of an exceptional and historic circumstance owing to an unprecedented rise in water levels causing flooding. The safety of residents remains our biggest priority, and that's why I declared the state of emergency," Montreal Mayor Denis Coderre said.
"I ask all residents affected by the flooding to collaborate by following the recommendations and abiding by the decisions made now and as we move forward. I strongly urge residents of Île Mercier and other residents whose safety cannot be ensured by emergency services to comply with the evacuation order," he said.
The state of emergency applies to the areas of Ahuntsic--Cartierville; Île-Bizard--Sainte-Geneviève; Pierrefonds-Roxboro; Sainte-Anne-de-Bellevue; and Senneville.