The assault occurred Saturday at about 4:30 a.m. in the 2200-block of North Orchard Street. According to police, a man approached two women standing in the alley, shoved them to the ground and sexually assaulted one of them.
One of the women was able to fight the man off by kicking him in the face.
Police have released a surveillance image of the suspect, who was riding a bicycle. Police said the suspect is in his late 20s to early 30s, 5'5"-5'6", 150 pounds, with short dark hair and may have facial lacerations. He was last seen wearing a white short-sleeved T-shirt and shorts.
Anyone with information is asked to contact Area Central detectives at (312) 747-8380.