The girl told police the man had smacked her face three times and driven off, Senior Sergeant Brian Benn said last night.
She said the incident happened in Glenross St, between 12.30pm and 1pm, as she was walking home from school.
The girl had given a detailed description of her assailant, Snr Sgt Benn said - a male caucasian in his late 20s, of slim build, sporting a moustache, wearing a red hooded sweatshirt and light green trousers, and driving a red four-door car, which was in need of a clean.
Police were keen to hear from anyone who recognised the man or the car.
The girl continued home and told a relative, who called police.
Officers patrolled the area but were unable to locate the man or the vehicle.
The girl had some facial redness, the policeman said.
