Gastric duplications are rare digestive tract anomalies and usually occur from the greater curvature of the stomach. We report an uncommon case of a gastric duplication mimicking a retroperitoneal cyst, which could not be directly identified during laparotomy. A 10-year-old girl was referred to our institution with intermittent upper abdominal pain. MRI showed a cystic mass measuring 62 mm in diameter located between the left kidney and spleen. There seemed to be no communication with the surrounding organs. Serum levels of tumor markers of adrenal gland tumors were within normal range. Surgery was carried out for suspected left retroperitoneal cyst. The cyst did not exist in the retroperitoneal space, but developed on the gastric fundus. The cyst was excised without any injury to the gastric muscles. The cyst contained dense white fluid. Pathologically, the inner wall of the cyst was lined with gastric mucosa and ciliated columnar epithelium.