The fire started in the elderly victim's fifth-floor bedroom inside the 4th Avenue building just after midnight.
Firefighters put the fire out very quickly, but they made the tragic discovery once the flames were extinguished.
The 84-year-old woman was found unconscious in the apartment. Firefighters tried to revive her but were unsuccessful.
The fire, at a building known as the Petersfield, was under control just before 1 a.m. The cause is under investigation, but the fire is not believed to be suspicious.