The Fukushima Daiichi nuclear power plant (FDNPP) accident in 2011 primarily caused indirect adverse effects. Even the indirect adverse effects of radiation/nuclear disasters sometimes lead to death. Japan compensates for disaster-related deaths under the legally enshrined disaster condolence grant system. Studies on disaster-related deaths in areas heavily affected by radiation/nuclear disasters have been limited. We investigated 520 fatalities in Minamisoma City, Fukushima Prefecture, located near the FDNPP, that were certified as disaster-related by the Minamisoma City Committee for Certification of Disaster-Related Deaths. The most common reason for certification of death as disaster-related was ‘displacement owing to evacuation’ (25.8%) followed by ‘lack of appropriate treatment’ (20.4%). Disaster-related deaths that occurred more than 6 months since the incident accounted for 37.8%. According to the Nagaoka criterion, which is one of the current guidelines for disaster-related deaths in Japan, a death must occur within 6 months to be recognised as disaster-related. However, the results of this study suggest that delayed deaths occurring more than 6 months after the incident may still be disaster-related particularly when involving nuclear/radiation disasters. Compared to common disasters, radiation/nuclear disasters may have more long-term effects on affected people owing to measures such as evacuation. This suggests that long-term measures are needed to prevent unnecessary deaths.