We describe herein the findings from postmortem computed tomography (PMCT) of barium peritonitis due to descending colon perforation. The patient was a woman in her 60s who underwent upper gastrointestinal series with barium swallow for the purpose of physical examination. The patient developed abdominal pain the next day and visited a clinic, but was sent home for later follow-up. She was found dead at home 8 days after upper gastrointestinal series. Based on the corpse phenomena and police investigations of the scene of death, the patient was estimated to have died 6 days after the upper gastrointestinal series. PMCT revealed free gas within the peritoneal cavity. Barium and fat stranding were also observed around a diverticulum in the descending colon. A large amount of residual barium was seen in the ascending colon and was considered to represent antemortem constipation and delayed barium excretion. No gas was detected in the abdominal wall, cardiac chambers or hepatic vasculature. The pancreas and adrenal glands had a normal appearance. We diagnosed barium peritonitis due to descending colon perforation as the cause of death. Diverticular perforation was suspected because barium retention and fat stranding were particularly noticeable around the descending colon diverticulum. In conclusion, we have presented a rare case of postmortem diagnosis of barium peritonitis. In PMCT interpretation, distinguishing between ante- and postmortem intestinal perforations is important.