Emergency crews responded around 1 p.m. to the area near 63rd Street Beach on the city's South Side, police said.
The man, identified as James Hammond, and three children -- his nieces and nephew -- were in a boat when the 11-year-old boy fell into the water, fire officials said. Hammond jumped into the lake to help the child, but went under.
The child, who was wearing a life jacket, was rescued by Chicago Park District lifeguards who rowed out to help, police said. Two children in the boat got the attention of the lifeguards. They were not hurt.
"Lifeguards on the beach heard the commotion and were able to quickly row out there in a lifeguard rowboat and assist the 11-year-old who was in the water," said Ron Dorneker, deputy district chief for the fire department.
A helicopter dive team pulled Hammond out of the water.
Hammond was rushed to University of Chicago Medical Center in "extremely critical" condition where he died, officials said. The child was transported to Comer Children's Hospital in good condition.
Hammond owned the small powerboat, family said.