We report a case of undifferentiated carcinoma of the duodenum. A 40-year-old man was referred to our hospital as he experienced upper abdominal pain that had persisted for half a year. Gastrointestinal endoscopy revealed a semicircular tumor in the duodenum. Enhanced computed tomography( CT) revealed that the tumor had not invaded the adjacent tissues, and lymph node metastases were not detected. Biopsy indicated a diagnosis of Group V cancer. Because the tumor was diagnosed as primary duodenal cancer, pancreatoduodenectomy and lymphadenectomy were performed. Histopathological examination revealed the presence of variant cells with irregular cores; we then diagnosed the tumor as undifferentiated cancer of the duodenum. No lymph node metastasis was detected microscopically. This patient has remained well without recurrence for 17 months since the operation. Undifferentiated carcinoma of the duodenum is rare, as only 9 cases have been reported in the Japanese literature.