Police said two sisters, 15 and 6 years old, were walking in the 4000-block of North Elston Avenue at about 9:15 p.m. when a man riding a red bicycle approached them.
The man exposed himself to the girls and grabbed the 15-year-old girl by the jacket, police said. The girl was able to break free and the girls fled.
The man was last seen riding his bicycle southbound on the 3900-block of North Bernard Street. The suspect is described by police as a man in his late 30s or early 40s with short black hair and a mustache.
Anyone with information is asked to contact the Chicago Police Special Investigations Unit at (312) 492-3810.