Montreal police said the fire started shortly before 9 p.m. at the Cabaret Playhouse on the building's second floor.
A Japanese restaurant on the building's first floor was also damaged.
Two customers and one employee were in the bar at the time.
The employee was treated in hospital for respiratory problems.
Police didn't release details about the circumstances surrounding the fire, but said it appears accelerant was used.
The arson squad is investigating.