A 63-year-old man was admitted to Saitama-Kyodo Hospital for the management of obstructive jaundice. Gastrointestinal endoscopy revealed a tumor of the duodenal papilla, and tumor biopsy suggested adenosquamous cell carcinoma. Computed tomography revealed multiple tumors in both the liver and lung, and these were diagnosed as metastases using bronchoscopy-guided lung biopsy and ultrasound-guided liver biopsy, respectively. The patient was treated with gemcitabine therapy after successful management of the jaundice by percutaneous transhepatic cholangiodrainage. However, he died three months after hospitalization. Autopsy confirmed a tumor of the duodenal papilla that had invaded both the pancreas and bile duct; moreover, multiple liver and lung metastases were observed. The pathological diagnoses were adenosquamous cell carcinoma. Histopathological findings revealed a mixture of adenocarcinoma and squamous cell carcinoma and promotion of multiplication of the adenocarcinoma. Adenosquamous cell carcinoma of the duodenal papilla is rare and preoperative diagnosis is challenging. Our case is unique because biopsy suggested the diagnosis before treatment.