The man competed in the Ironman distance at Challenge Almere-Amsterdam, just east of the Dutch capital.
He reportedly fell ill during the opening swim, the local public broadcaster Omroep Flevoland reports.
Although rescue personnel immediately pulled the man out of the water, his life could not be saved; he later died in hospital.
"This leaves deep wounds, for us as the organiser but above all for the bereaved," the competition organiser writes in a statement.
