Firefighters said the blaze began around 2:15 a.m. in an apartment on Danforth Ave., near Greenwood Ave., and flames could later be seen shooting from the apartment.
Toronto Fire Services divisional commander Bob O'Hallarn said a passerby alerted firefighters are noticed smoke coming from the building.
O'Hallarn said firefighters found the victim "obviously deceased" inside the apartment's bedroom, where the fire appeared to have broken out.
Nobody else was injured or required rescuing from the building, O'Hallarn said.
It's not immediately clear what caused the fire.
The Ontario Fire Marshal's Office, which investigates all fatal fires, has been called to the scene.