Chicago police said officers responded to a wellbeing check in the Gladstone Park neighborhood's 5400-block of North Menard Avenue around 1:30 p.m. Tuesday.
A witness on the scene found an unresponsive 7-year-old girl in the pool, police said.
Police said the girl was transported to Lutheran General Hospital in serious condition.
The Cook County Medical Examiner's Office said the girl, identified as Leen Awad, was pronounced dead on Thursday.
Authorities say the drowning is classified as an accident.
Further information was not immediately available.