We report an unusual case of a polyp arising from ectopic gastric mucosa in the duodenum. The polyp demonstrated both hyperplastic and adenomatous proliferation. Adenomatous proliferation in a duodenal polyp arising from ectopic gastric mucosa is, to our knowledge, unique. The areas of adenomatous change were morphologically indistinguishable from those of adenomatous polyps arising in the stomach; hence, the adenomatous change observed in this polyp was regarded as a neoplastic process.