Joliet police said officers responded to the 400-block of Market Street just before 5 p.m.
There, officers found a 13-year-old boy who had been shot in the shoulder.
The Joliet Fire Department transported the boy in critical condition to Saint Joseph Medical Center, and he was later airlifted to Loyola University Medical Center in Maywood.
Detectives canvassed the area. What led up to the shooting was not immediately clear.
No one is in custody, and police are investigating.
Joliet police asked anyone with information to contact them at 815-724-3020. Those wishing to remain anonymous may contact Will County Crime Stoppers at 800-323-6734 or submit a tip online at crimestoppersofwillcounty.org.