Emergency crews responded to a call at 304 Sorauren Ave. at about 10:30 p.m.
Toronto police were the first to arrive at the scene, but the intensity of the flames forced them to pull back and wait for firefighters.
Once firefighters gained access to the apartment the fire, which was contained in the second-floor unit, was quickly put out.
The body of a woman was discovered lying on a couch in the living room.
Jason Rice was outside walking with a friend when he noticed thick smoke coming from the duplex.
"Once we saw [smoke] coming from the side of the house, I was slamming on some doors, knocking on some doors," he said. "The owner came out, we got some keys and got through the door and he told me that there was somebody up there but we couldn't get much further than five feet."
Firefighters told CBC News they found a smoke detector in the unit, but it appears the batteries had been removed.
There is no word what may have caused the fire. The Ontario fire marshal is investigating.