The 11-year-old boy whose body was pulled from a canal had been "tombstoning", South Yorkshire Fire and Rescue said.
The schoolboy, who has not yet been named, jumped 18ft into the canal near Rotherham.
The fire service warned others to stay away from canals and not to take part in tombstoning - jumping from unknown heights into unknown depths of water.
Police were called to the canal at around 7pm Thursday following reports that the boy had been seen entering the water and disappearing soon after.
Search teams found the body at around 11pm and the victim's family has been informed.
A South Yorkshire Police spokesman said: "Following an extensive search of a Rotherham Canal yesterday, we can confirm that specialist teams have recovered the body of an 11-year-old boy.
"Police received a report at around 7pm that a boy had been seen entering the canal, off Stone Row Way, but had disappeared soon after.
"Specialist resources attended the scene, including South Yorkshire Fire and Rescue and the ambulance service. The body of the 11-year-old was found in the water at around 11pm.
"His family has been informed and are being supported by officers.
"The circumstances are under investigation."