The fire broke out at about 1 a.m. on the first floor of the building on John F. Kennedy Street.
Firefighters say it quickly spread to the second floor and the roof.
Montreal police spokesman Raphael Bergeron said cannabis was found in the building.
"They discovered a plantation," Bergeron said.
One person went to hospital with burns and another man was arrested.
In all, five people were forced from the building.
Bergeron said firefighters are still trying to determine the cause of the fire. The investigation has been handed over to police.