An 86-year-old Japanese man was presented to our hospital for further investigation of duodenal adenocarcinoma. The tumor was endoscopically resected. Pathological analysis revealed coexistence of gastric foveolar metaplasia and a surrounding hyperplastic Brunner's gland, in addition to an adenocarcinoma component. Immunostaining for MUC5AC and MUC6 confirmed the diagnosis of adenocarcinoma in situ arising from Brunner's gland hyperplasia. This case suggests that although detailed preoperative evaluation is required to determine the depth of tumor invasion, endoscopic resection may be a promising option for the treatment of adenocarcinomas arising from Brunner's gland hyperplasia.