A 28-year-old woman with ascites was admitted to our hospital. We diagnosed peritonitis carcinomatosa caused by colon cancer complicated by ulcerative colitis. We performed peritoneal tap and infusion of mitomycin C, and administered 5-fluorouracil. Her clinical status gradually worsened, and she died 5 months later. At autopsy, the histological examination showed many mucinous adenocarcinoma and signet ring cell carcinoma with dysplasia. There were also some areas of squamous cell carcinoma with squamous metaplasia and dysplasia far from rectum. Squamous cell carcinoma and adenosquamous cell carcinoma of the colon are rare complications of ulcerative colitis. We reported this case as an addition to the literature on the subject.