The Mexican Triathlon Federation announced "with deep sadness" on X that Roger Mas Colomer, 79, died while competing at the event in Torremolinos, near Malaga on the Mediterranean Costa del Sol.
British Triathlon said in a statement that "one of our Age-Group Team has sadly passed away", referring to the sub-discipline for veteran competitors.
World Triathlon confirmed the deaths of two athletes on social media and said: "Our deepest condolences go to their respective family, friends, National Federations and all the triathlon family."
Local media reported the Briton was a 57-year-old man who died following a heart attack.
More than 4,000 triathletes had signed up for this year's age-group event for older athletes, out of more than 5,500 taking part in the entire competition, the Spanish Triathlon Federation said last month.