Gastric teratomas are extremely rare neoplasms that have clinical characteristics such as a male predominance and a benign nature. We experienced a case of mature gastric teratoma occurring in a 12-month-old boy, who presented with an exogastric cystic mass, and this was confirmed by complete excision with subsequent pathologic examination. We report here on the radiologic and pathologic findings of an exogastric mature teratoma in an infant.