The man, who was not wearing a life jacket, was pulled from the 59th Street Harbor on Chicago's South Side, according to a 9:08 p.m. tweet by the Chicago Fire Department.
He was transported to the University of Chicago Medical Center. He was under water for more than 15 minutes.
The man's condition was not immediately released by officials.
The victim fell from a jet ski while "partying," according to a fire department tweet.
In a tweet, the fire department urged jet ski users to wear life jackets.