Ottawa Fire crews were called to the site of the former Overbrook Public School late Monday night after reports of people smelling smoke in the area.
They discovered a fire inside the three storey building at 149 King George Street and got the blaze under control quickly.
No injuries were reported.
The old school is currently under renovation and was empty at the time of the fire.
Damages are estimated at $50,000.
The cause of the fire is under investigation.