The fire broke out on the third floor of the building on First Avenue at 91st Street just before 5:45 a.m.
The 86-year-old woman, identified as Audrey Bergensten, was pronounced dead inside the bathroom of the apartment.
"Other than that victim, thankfully no other injuries, either to my members or anyone else in the building," FDNY Deputy Chief Nicholas Corrado said.
The fire remained contained to the victim's unit.
Fire marshals say the fire was caused by an extension cord. The FDNY said a smoke alarm was present, but it wasn't working.
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