It happened at the Livingston Building on the 1400 block of Hamilton Street around 7:30 p.m. Tuesday.
Witnesses told WFMZ-TV that a woman performed CPR on a 5-year-old girl who was then rushed to a nearby hospital by an officer.
5-year-old girl falls from 4th-story window in Allentown
The girl was later pronounced dead, according to the Lehigh County coroner.
Police said the incident appears to be accidental.