Firefighters evacuated some of the residents using ladders and at least one firefighter and a resident were reportedly injured during the operation.
About 80 people have been forced out of the building and the local city hall has been opened to accommodate them.
Repentigny fire department spokesman Marc Rocheleau says a cooking fire was the likely cause of the blaze.
Firefighters were also battling a stubborn blaze early Wednesday in a duplex in the Notre-Dame-de-Grace district in west-end Montreal.
The five-alarm fire, which began at about 3 a.m., forced the evacuation of about 20 people from adjacent buildings. One woman was taken to hospital suffering from shock.
There was no word on the cause of the fire, which was still being fought by 6 a.m.