The body was spotted near the 800 block of Des Plaines Avenue in Forest Park Tuesday.
Maywood police have taken over the case. An autopsy is scheduled for Wednesday morning.
A spokeswoman with the medical examiner's office tells the Chicago Tribune it isn't clear how long the body was in the water. But she says it was decomposed to the point that the person's age and race couldn't immediately be determined.
The Associated Press contributed to this report.