The shooting happened outside the 79th Street Red Line station in Chatham at about 4:15 p.m., police said. Two people, including a 17-year-old boy, fired shots at each other.
The teen was shot in the upper body and transported to the University of Chicago Medical Center in critical condition, police said.
The other person, who is male, fled the scene, police said. Area Two Detectives are investigating.
The CTA said Red Line trains are running, but are not stopping at 79th Street due to police activity.
Authorities have not provided additional information about the incident.
On Saturday, a man was shot and killed while on a Red Line train at 79th Street, police said.