It happened around 5 p.m. Saturday on the 500 block of East Girard Avenue.
Police say the boy, roughly 5 years old, was in a bedroom playing with two siblings, a 7-year-old and 11-month-old, when he fell out of a second-story window.
Police say the child's mother was home at the time when the bedroom window screen gave way.
It is unclear where the mother was inside the apartment or if she was with the boy.
There were multiple witnesses outside of the building who saw the child at the window. One bystander was able to break the child's fall.
The boy was airlifted to an area hospital where he was placed in extremely critical condition.
No charges have been filed at this time.