Duplication cysts are uncommon congenital malformations that may occur anywhere throughout the alimentary tract. The stomach is an extremely rare site of occurrence. Here, we report a case of gastric duplication cyst initially presenting with a gastric submucosal tumor. A 28-year-old man complained of dyspepsia lasting 1 year and upper endoscopy revealed an ellipsoid submucosal tumor at the greater curvature of the antrum. We intended to use the injection-and-cut technique: however, after saline injection, the lesion was dented and impossible to grasp with a snare. Therefore, we decided to perform endoscopic submucosal dissection and removed the tumor without complication. Histopathology revealed a 0.6×0.6 cm-sized duplication cyst, and there has been no recurrence in 2 years.