Police say the officer on duty and was responding to a call on the city's South Side at around 12:30 a.m. in the 5700-block of South Winchester Avenue.
Accroding to police, the officer was trying to break up a fight in the middle of the street. One man allegedly hit the officer in the head with the bat repeatedly after the officer got out of his car.
More than a dozen officers responded to the scene to investigate later.
The injured officer was being treated at Stroger Hospital. No one was in custody later Saturday morning.
Area South detectives say they are investigating.