The tragic incident unfolded as the athlete took part in the swim section of the event just after 9am on Sunday.
Activity Wales Events, the race organisers, said it was “with heavy hearts” that it confirmed the death as it passed on its sympathies to the athlete’s family.
The Welsh Ambulance Service, which was supported by St John’s Ambulance, wrote on Facebook: “It is with heavy hearts that we confirm the passing of an athlete during the swim section of today’s Triathlon.
“Our most sincere condolences go out to the family.
“We would like to thank the incredible efforts of the swim safety team, St John’s, Ambulance, the rapid response team, the police, volunteers and members of the public involved at the scene.”
Up to 1,500 people were entered in the University of Wales Trinity Saint David Swansea Triathlon which included a 750m swim, 20km bike ride and 5k run.
A Welsh Ambulance Service spokesman said: “We were called at approximately 9.15am to a medical emergency in Swansea.
“We sent a rapid response vehicle, an emergency ambulance, and the Emergency Medical Retrieval and Transfer Service. We were also supported by St John’s Ambulance.”
