Small intestine cancers are uncommon diseases among gastrointestinal cancers. Among these small intestine cancers, adenocarcinoma is the most common type. Other types of cancers often found in the small intestine are carcinoid, lymphoma, sarcoma, and gastrointestinal stromal tumor. Adenosquamous carcinomas of the duodenum are extremely rare lesions among small intestine cancers, and only a few cases have been reported so far. In these reported cases, tumors arose from the ampulla of Vater, periampullary portion, or duodenal fourth portion. Never has an adenosquamous carcinoma been reported to arise from the duodenal third portion. Herein, we present such a case.