With the aim of extracting lessons learned, this study investigated a large oil spill at an ironworks factory in Saga prefecture, during the severe flooding that hit southwestern Japan in late August 2019. The oil spill dispersed by the flood waters contaminated adjacent crops, irrigation canals and citizens’ homes in a large area of Omachi town. Many citizens had practiced vertical evacuation. Due to the oil spill, the pumping of flood waters had to be stopped to prevent further contamination, resulting in oil stagnating in the area for several days. This meant that residents had to be rescued from their homes in the middle of strong oil vapours. The oil spil’s possible long-lasting impact in terms of health and environmental pollution requires monitoring and further investigation. The study found that oil spills caused by floods had already occurred at the same site, highlighting the need to improve risk management of chemical hazards, develop flood risk maps that consider the potential for these types of secondary events and other compound disasters, and propose more effective strategies for emergency planning and response.