The 16-year-old student rode the southbound train from Jarvis to the stop at State and Jackson, where she tried to exit the train car about 7:15 a.m., according to a community alert.
A man grabbed the girl inappropriately, and she told him to stop, which made several passengers on the train take notice, police said.
The man remained on the train. The girl went to school, where she told police what happened.
The man is described as black, about 35 years old, standing 5-foot-8 and weighing 160 pounds. He was wearing an oversized black jacket, hood with fake fur and black jeans.
Police ask anyone with information to call 312-747-8380.