"It is our understanding that this is the first death to ever to take place at the Escape from Alcatraz Triathlon, which has taken place for 33 years," race director Bill Burke said in a statement.
The name of the man, who was from Austin, Texas, was not released, KGO-TV, San Francisco, said.
Spain's Javier Gomez won the event in a time of 2 hours, 4 minutes.
Rosie O'Donnell unveils nearly 50-pound weight loss
Putin thinks Obama would save him if he were drowning