The crime was reported along Germantown and Erie Avenues around 9:30 p.m. Friday in the city's Tioga-Nicetown neighborhood.
Police say the 12-year-old was sleeping inside a running 2010 Ford Taurus when a suspect entered the vehicle and drove off.
A half hour later, the girl was found near Broad Street and Roosevelt Boulevard. Police say the child told officers the suspect left her there after a sexual assault.
The suspect is still on the loose, and the vehicle has not yet been recovered.
Police say the vehicle has a Delaware temporary plate with the tag number: "XF240151" and appears to be missing a rear hubcap and fuel tank cover.
Anyone with information is asked to call police at 215-686-TIPS.