The incident occurred at about 8:30 a.m. in the Chatham Ridge Shopping Center in the 100-block of West 87th Street.
Police said a 23-year-old man working as a security guard became involved in a verbal and physical altercation with a large group of individuals.
The security guard was struck and suffered a broken finger and fired his weapon, hitting a 28-year-old man in the wrist and ankle.
The wounded man was transported to the University of Chicago Medical Center in serious but stable condition, police said.
The security guard has a valid FOID and CCL, police said.
No other injuries were reported and Area Two detectives are investigating.
INTERACTIVE SAFETY TRACKER Track crime and safety in your neighborhood