Police say the officers saw a man fire a weapon at a group of men. They announced that they were police and ordered the man to drop his weapon. Instead, they say, the suspect turned in their direction.
After a second order to drop the weapon, one of the officers fired, striking the man in the thigh. He was taken to Stroger Hospital in stable condition.
A gun was recovered at the scene, police say.
Area Two detectives and the Independent Police Review Authority are investigating.