The on-duty officer was dressed as a civilian and on the sidewalk conducting an investigation at about 11:25 p.m. in the 5800-block of South Loomis Boulevard when someone inside a black van fired a paintball gun, police said.
The officer was hit in his left side but was not injured. Police pursued the vehicle to Calumet Park, where three male suspects fled on foot.
One suspect was taken into custody and no injuries were reported. Area South detectives are investigating.