Multiple 911 calls were made after the man was battered near the intersection of Roscoe and Inner Lake Shore Drive at 3:45PM.
Officers believe the offender may have psychiatric issues.
The suspect is described as a blonde-haired white man with a beard. He was wearing khaki pants and was shirtless when last seen walking westbound on Roscoe.
The victim was transported to Illinois Masonic Medical Center for treatment.
Image: Google Street View photo of Roscoe at Lake Shore Drive.