We report a case of mucosal duodenal cancer in a 62-year-old woman, which was successfully removed en bloc by endoscopic submucosal dissection (ESD). The patient underwent an upper gastrointestinal endoscopy at our hospital, which revealed an elevated flat mucosal lesion (type IIa) measuring 10 mm in diameter in the second portion of the duodenum. Histopathological examination of a biopsy specimen revealed features suggestive of a tubulovillous adenoma with severe atypia. As the findings suggested that the lesion had an adenocarcinoma component but was confined to the mucosal layer, we decided to carry out ESD and successfully removed the tumor in one piece. The resected tumor was 20 x 15 mm in size. Histopathological examination revealed that the lesion was a well-differentiated mucosal adenocarcinoma with no lymphovascular invasion. Mucosal duodenal cancer is extremely rare, and ESD of a lesion in the duodenum requires a high level of skill. To the best of our knowledge, this case is the first report of successful ESD carried out in a case of mucosal duodenal cancer.