The fire began just after 1 a.m. ET Thursday in a mixed residential and business area of the Gay Village.
The flames gutted the second floor of the building.
A Pizza Pizza restaurant as well as at least one other business were heavily damaged by water and smoke.
One firefighter was taken to hospital to be treated for smoke inhalation.
Ste-Catherine Street East will remain closed to traffic between Amherst and St-André streets Thursday morning.