Gastric duplication is a rare congenital anomaly accounting to 4% of all gastrointestinal tract duplications. It is usually diagnosed pre-natally or during infancy. Retroperitoneal gastric duplication is very rare in adult patients. It is generally asymptomatic or presents with non-specific symptoms. The sensitivity of classical imaging modalities for retroperitoneal gastric duplication is weak. Retroperitoneal gastric duplication could be misdiagnosed as other retroperitoneal cystic lesions and it could be definitely diagnosed by typical findings of abdominal enhanced computed tomography (CT) and endoscopic ultrasonography (EUS) plus EUS:guided fine needle aspiration biopsy in gastric duplicated cyst. A female adult patient was diagnosed by an exceptional hospital CT as retroperitoneal cystic lesion and was admitted in our hospital in March 2019. The abdominal enhanced CT in our hospital preoperatively indicated a cystic lesion above the left kidney and the maximum cross section was 9.0 cm ×5.2 cm. Lymphocysts and others should be identified and the lesion might be from the stomach. The retroperitoneal gastric duplication was not diagnosed before operation. In the operation laparoscopy revealed the cystic lesion in the retroperitoneum. The pancreas, left adrenal gland, and left kidney were compressed by the cystic lesion. The top of the lesion was connected with the small curvature of the stomach, but it was not communicated with the stomach. The retroperitoneal cystic lesion was completely excised and the surrounding organs were not damaged. The retroperitoneal gastric duplication was clearly diagnosed by the findings in the operation and the pathology of the lesion. The patient was discharged a week after surgery without gastrointestinal and pancreatic injuries. Therefore, the differential diagnosis of retroperitoneal cysts should consider the possibility of retroperitoneal gastric duplication. For patients with typical CT findings or unclear boundaries between the tumor and the stomach wall, gastric duplication could be diagnosed by EUS plus EUS:guided fine needle aspiration biopsy in the cyst. Retroperitoneal gastric duplication cyst could be completely excised by laparoscopy through abdominal cavity and the stomach wall could be completely repaired.