An 80-year-old man who had a rectosigmoid cancer on colonoscopy for melena was admitted to our hospital for surgery. A type-2 lesion of the ascending colon was also detected by preoperative colonoscopy, and Hartmann operation and right hemicolectomy were carried out. Histologically, the rectosigmoid cancer was undifferentiated cancer and the ascending cancer was highly and moderately differentiated adenocarcinoma. He was discharged on the 24th postoperative day. However, on the 43rd postoperative day, he was readmitted to our hospital due high inflammatory findings on a blood test. His lower abdomen was extremely distended and abdominal CT examination revealed a huge pelvic mass due to the recurrent tumor and multiple hepatic metastases. He died because of cancer cahexy on the 73rd postoperative day without a chance of chemotherapy. We herein report a rare case of undifferentiated carcinoma of the rectum with a review of the literature.