Chicago police say the incident happened just before 1 a.m. on Sunday, near 24th and Rockwell in the Little Village neighborhood.
Authorities say the officer had stopped his vehicle and was verbally engaging with a group of people standing on the sidewalk when a man from the group approached and physically confronted the officer.
The off-duty officer fired his weapon, striking the man twice.
He was taken to Mount Sinai Hospital and remains in police custody.
A second man standing in the group was also injured.
After the initial shooting, police say the officer drove a short distance before stopping his vehicle and shooting in the direction of the group.
He was taken to the hospital with injuries he sustained during the physical altercation.
The Civilian Office of Police Accountability is investigating.
CPD says the off-duty officer was placed on administrative duties for 30 days.
