The officer was struck in the head with the bat while trying to break up a fight early Saturday morning in the 5700 block of South Winchester Avenue in West Englewood.
The officer was taken to the John H. Stroger Jr. Hospital of Cook County in serious condition.
Authorities are now speaking with "person of interest" in connection with the attack.
So far, no charges have been filed.
Sun-Times Media Wire contributed to this story.