Organisers said the participant suffered a serious medical emergency during the 253-mile Arizona race, which began on Monday and runs until Saturday from Black Canyon City to Flagstaff.
Aravaipa Running said no personal details would be released out of respect for the family, and the event would continue in the runner's honour.
Nearly 400 runners were listed on the race tracker. The death follows other fatal ultramarathon incidents, including one in Colorado in 2025 and 21 deaths in China in 2021.
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