DNP (2,4-dinitrophenol) has been commonly used as a weight loss aid; in particular, it has been used by body builders to achieve significant fat loss while maintaining muscle mass. The sale of DNP was banned due to some cases of fatal poisoning. We describe an autopsy case of a 39-year-old man who died from taking DNP to treat obesity. He was 183 cm tall and weighed 101 kg (body mass index of 31). External examination did not reveal any specific findings implicating the cause of death. Gross examination revealed that the heart weight was 450 g, and there were no significant findings in the coronary arteries and myocardium, except for a few petechiae of the epicardium. The DNP concentration in the subject’s blood was 38.28 mg/L. According to references, a DNP blood concentration of 28 mg/L or higher would result in death. Therefore, the cause of death in this autopsy case was DNP toxicity.