Planning the medical services for the triathlon competition at the 2020 Tokyo Olympic and Paralympic Games was predicted to be challenging because of possible last-minute changes related to the COVID-19 pandemic and abnormally high temperatures. Therefore, event planners organized and executed a disaster medical care plan, a plan for providing care during emergencies. Based on the basic medical plan for all venues provided by the Tokyo 2020 Organizing Committee, planners for the triathlon venue prepared a medical care plan according to the CSCATTT principles: Command and control, Safety, Communication, Assessment, Treatment, Triage, and Transport. After the event, planners evaluated the number of COVID-19, heatstroke, and injury cases at the venue. The events were conducted without spectators in July and August 2022 because at the last minute, planners held the event without spectators. The triathlon competition involved 638 individuals, including athletes and staff. In total, 7 cases of injuries, 3 cases of mild heatstroke, and 13 other cases were reported, with only 2 requiring emergency transportation. No cases of COVID-19 were reported from the triathlon venue, including during the observation period after the event. This medical plan was effective in preventing heatstroke and COVID-19 cases during the big event. Efficiently and effectively responding to various situations is possible in a shorter period by planning large-scale medical services for such special circumstances according to CSCATTT, a principle of disaster medical care.