District officials responded to a report of a water rescue near Burton's Bridge on Route 176 in McHenry County just before 2 p.m.
One person was reported missing, and an overturned jet ski could be seen in the middle of the Fox River, officials said.
The missing person's body was found in the water just after 3 p.m.
A second jet skier, who was traveling in tandem on a separate watercraft, declined medical treatment at the scene.
The Illinois Department of Natural Resources is investigating.
Further information was not immediately available.