This report describes a case of carcinosarcoma of the duodenum. Carcinosarcoma of the duodenum is a very rare tumor. A 72-year-old man was referred to the hospital because of appetite loss. Endoscopy demonstrated an irregularly depressed lesion (type 3) in the descending portion of the duodenum opposite to the ampulla of Vater. Computed tomography showed a thickened duodenal wall and swelling of the abdominal para-aortic lymph nodes. A biopsy specimen revealed a well-differentiated adenocarcinoma. A diagnosis of duodenal carcinoma was made (cT3, cN1, cM1, cStage IV according to the TNM classification). A subtotal stomach-preserving pancreatoduodenectomy and a lymph node resection were performed. On microscopic examination, adenocarcinoma cells and spindle type sarcoma cells were observed separately in the descending portion of the duodenum opposite to the ampulla of Vater. The adenocarcinoma cells were stained with antibodies against epithelial markers keratin and carcinoembryonic antigen for immunohistochemical analyses. In contrast, the sarcoma cells were stained with antibodies to vimentin and smooth muscle actin. The pathological diagnosis of a true duodenal carcinosarcoma was thus made.