A competitor passed away during the swim leg of the Sunsmart Busselton Ironman 70.3 on Saturday.
During the team's category of the leg, the 39-year-old man was seen by surf lifesavers as needing assistance.
The individual was attended by the surf lifesavers who treated the patient and administered CPR under the direction of St John Ambulance personnel and the event doctor.
The patient was then transferred to the Busselton Health Campus for further treatment.
When the patient arrived at the hospital, he had passed away.
Event organisers released a statement saying there would be more investigations into the incident.
"We are all very sad to have had this happen today and offer our condolences to the family and friends of the deceased," the statement said.
The 39-year-old man is believed to be from the Perth suburb of Winthrop.
Thousands of competitors were in Busselton for the Sunsmart Ironman 70.3 which consists of a 1.9 kilometre swim, 90km ride and a 21.1km run.
Sam Appleton and Felicity Sheedy-Ryan took first place honours in the 2015 event.
