The measure was declared on Sunday afternoon and will remain in place for 48 hours, city officials said.
Across the province of Quebec, in Canada’s east, nearly 1,900 homes have already been flooded across 126 towns and cities, authorities say.
Some 1,200 troops have reportedly been deployed to help evacuate people.
“Our troops are responding quickly and professionally and are already beginning to deliver critical support to Canadians affected by the flooding,” Defence Minister Harjit Sajjan said.
The emergency was declared in Montreal after three dikes gave way in the city’s north.
The town of Rigaud, west of Montreal, has been particularly hard hit and a state of emergency has also been declared there.
The mayor has ordered mandatory evacuations in flood zones.