The shooting happened around 10:15 p.m. local time at a basketball court in the city's Back of the Yards neighborhood, according to police.
Chicago fire officials said 10 adults, two teenagers and a 3-year-old boy were shot. Twelve of the victims were taken away from the scene by ambulance. The 13th victim walked into the hospital.
Officials said the child was in critical condition.
The 3-year-old's uncle says his nephew was hit in the cheek. A 33-year-old woman was also hit near her shoulder. At least 10 ambulances were called to the scene to transport victims to area hospitals.
The motive for the shooting is believed to be gang-related, investigators said. No arrests have been made.
Witnesses say the suspect is a man in dreadlocks who was driving a gray sedan when he opened fired in Chicago's Back of the Yard neighborhood near the park.
The shooting comes nearly three weeks after Chicago saw an outburst of violence over the Labor Day weekend that ended with eight dead and 20 others injured.