A witness, physician Donald Cain from Wisconsin, told the Daily Camera in Boulder that a man was pulled from the water on Saturday.
Ironman officials said the athlete died Sunday evening. The athlete was not identified.
A 70.3 triathlon, or half Ironman, involves swimming 1.2 miles, biking 56 miles and running 13.1 miles.
Michelle Walters, 34, of McCook, Nebraska, died after being struck by a vehicle during Boulder’s 2016 Ironman race while 40-year-old Brian Godlove of Fairfax, Virginia, died three days after a 2015 race from dehydration and muscle tissue breakdown.