Firefighters responded to the 3800 block of North 19th Street in Nicetown-Tioga, around 3:30 Saturday morning.
They say flames were shooting from a second floor apartment that they thought was vacant.
Firefighters beat back the fire within 15 minutes.
They soon found the body of a woman. She has not been identified.
Investigators say the apartment was under renovation and contained no smoke detectors.
The cause of the fire remains under investigation.
Fire Commissioner Lloyd Ayers urges everyone to have a working smoke alarm on each floor of their home or apartment.
If you feel you can't afford a smoke alarm, Ayers asks you to call 311 or 215-686-1176.