Studies have argued that regret promotes behavioral change, as it embodies a painful lesson. Even in the context of a disaster, regret may facilitate behaviors that mitigate individual disaster-harm such as earlier evacuation and disaster-prevention actions. Through two surveys conducted on the 2018 Japan floods, we investigated (i) whether people experienced regret immediately after the flood disaster and (ii) whether such regrets would facilitate people’s harm-reducing behavior. In Study 1, the disaster victims were asked to recall their disaster experiences. As a result, we found that they regretted their failure to evacuate earlier and their lack of disaster-prevention behavior. In Study 2, approximately a year after the disaster, respondents reported how likely they were to regret their behavior at the time and how likely they had been engaged in potentially disaster-harm-reducing behavior. The results showed that regret predicted both evacuation acts and disaster-prevention behavior after the 2018 Japan flood. Further, we discuss future studies on regret and disaster-related behavior.