Hyperplastic polyps constitute 75% of all gastric polyps. Most of them are innocent lesions and are smaller than 1 cm. They develop as a consequence of an exaggerated mucosal response to tissue injury and inflammation on the basis of chronic gastritis. A 69-year-old male was admitted to the Gastroenterology Clinic with complaints of abdominal pain and vomiting. Gastroscopy showed a polyp that was 2 cm in its greatest dimension. A histopathologic examination of this lesion revealed adenocarcinoma infiltration in the mucosa. Subtotal gastrectomy was performed. Macroscopically, a polyp with a stalk was seen in the antrum; microscopically, it was consisted with a hyperplastic polyp. In the polyp, a few foci of infiltrative glands embedded in the mucinous matrix were noticed in the lamina propria, and the patient was diagnosed as intramucosal mucinous adenocarcinoma arising from a gastric hyperplastic polyp. Because of the risk of carcinoma development, which is rare, all hyperplastic polyps should be completely excised, and a histopathologic examination should be carefully performed.