The blaze was called into Toronto Fire just after 5:30 p.m. ET.
Firefighters and two dozen vehicles were sent to the Canadian Tire in the RioCan Leaside Centre on the southeast corner of Eglinton Avenue and Laird Drive.
Flames and black plumes of smoke could be seen rising from the garden centre, located on the west side of the front of the building.
The blaze was brought under control by the early evening.
It damaged the exterior of the building and largely destroyed the garden centre, which housed an entrance into the main building.
David Gollom, manager of corporate communications for Canadian Tire, said no one was hurt and all customers and staff got out of the building safely.
"What we know is that it started in the garden centre, the sprinklers were activated, so everything worked as it should," Gollom told reporters on Monday evening.
He said the damage to the store is still being evaluated.