A 61-year-old man with chief complaints of upper abdominal pain and fever was referred to our hospital. Dilation of the gallblader and common bile ducts and a hemorrhagic tumor, 1.5 cm in size, in the duodenal papilla were detected, and poorly-differentiated adenocarcinoma was diagnosed based on biopsy. Based on a diagnosis of cancer of the duodenal papilla, subtotal stomach-preserving pancreaticoduodenectomy (SSPPD) was performed, and the results of histopathological examination, including immunostaining of the resected specimen, gave a final diagnosis of carcinosarcoma. The patient was discharged on postoperative day 38 and, based on his request, received no adjuvant therapy. On postoperative day 49, the patient visited our hospital with complaints of cold limbs and abdominal pain, and the onset of multiple liver, residual pancreatic, and lymph node metastases, as well as peritoneal dissemination, was evident. The patient had severe anemia due to intra-abdominal hemorrhage resulting from the recurrent tumor, and his condition rapidly deteriorated despite transfusions and other treatments. He subsequently died on postoperative day 63. Carcinosarcoma of the duodenal papilla is an extremely rare disease, having only been reported in five patients, including the present case. Although all patients were treated with surgery, three developed liver metastases and died within eight months following surgery, suggesting that this disease has a poor prognosis, as with carcinosarcoma of other organs.