Toronto Fire Services responded to a call at Da Leonardo Bistro, located at 1239 St. Clair Ave. W., just after midnight.
Crews arrived to find smoke pouring out the winds of the restaurant, which is located near the intersection of St. Clair Avenue West and Via Italia.
The fire had spread to the second floor, but firefighters were able to keep the flames from spreading to adjacent buildings.
Toronto Fire Services said the building was not occupied and not equipped with a sprinkler system. Toronto fire officials say it appears the fire started in the building's mechanical and electrical services room.
One firefighter was transported to hospital with an ankle injury.
The Fire Marshal's office is investigating.