The crime happened Wednesday on the 1100 block of South Lyman Avenue, police said.
Police say a man in his 20s was riding a bicycle when he approached the victim.
Investigators say he grabbed her from behind, there was a struggle and he placed his hands up her shorts.
There's only a vague description of the suspect and he was said to be wearing a black shirt and colorful shorts, riding a gray-colored bicycle.
Anyone with information has been asked to call the Oak Park police tip line at 708-434-1636 or contact Detective Smith of the Investigations Division directly at 708-358-5545 or tsmith@oak-park.us.