Crews responded to the scene, an apartment building in the 7500 block of 24th Avenue SW, at about 4:30 a.m. after receiving 911 calls about flames and smoke coming from a third-floor unit, said Seattle Fire Department spokesman Kyle Moore.
He said smoke was so thick in the apartment that firefighters couldn't see a hand in front of their face.
The woman's body was found in the bedroom next to a charred bed.
Moore said the building's sprinkler system worked, but only in the hallway and not in the bedroom.
Firefighters contained the fire to the bedroom, but the unit below sustained major water damage. There was no immediate word on the cause of the fire.
The name and age of the woman who died were not immediately available.
Moore says one resident of the building is displaced but others who were evacuated are being allowed to return.