The woman in her 60s or 70s died from asphyxiation after her mattress caught fire.
Montreal firefighters were called to the scene just after 9 a.m. and had the fire extinguished within a half-hour.
Firefighters found the woman dead in the hallway after breaking down the front door of the third-floor apartment.
The apartment did not have a functioning smoke detector.
Investigators are still trying to determine the cause of the fire.