The shooting happened around 4:01 p.m. in the 5200 block of South Wells Street in Fuller Park.
The 9-year-old boy was inside of a home with a firearm and fired one shot, police said.
A 13-year-old boy was shot in the shoulder area and was taken to a hospital in critical condition.
No further information was immediately available as Chicago police continue to investigate.
INTERACTIVE SAFETY TRACKER Track crime and safety in your neighborhood