Joliet police said its officers responded to Riverwalk Homes Apartments at 366 North Broadway Street just before 1 p.m.
There, officers found a 17-year-old boy, who had suffered a serious stab wound, on the fifth floor.
The Joliet Fire Department transported the victim to St. Joseph Medical Center, where he remains in critical condition.
Officers recovered a knife from the scene and brought a boy to the Joliet Police Department for questioning.
Joliet police are investigating and asked anyone with information to call them at 815-724-3020. Anyone wishing to remain anonymous can contact Will County Crime Stoppers at 800-323-6734 or submit a tip online at crimestoppersofwillcounty.org.
Further information was not immediately available.