INTRODUCTION: Duplication cysts are very rare congenital malformations in adults. They are lined by gastrointestinal mucosa, connect to the digestive tract, and share smooth muscular layers and a common blood supply. In rare cases, duplication cysts are completely isolated from the digestive tract and have a proper blood supply. Completely isolated duplication cysts in the retroperitoneum are unusual so it is hard to diagnose them without a surgical resection. PRESENTATION OF CASE: A 19-year-old male presented at our emergency department with sharp abdominal pain. Contrast-enhanced computed tomography detected a 5-cm multilocular cystic mass located in the retroperitoneum, caudal to the pancreatic body. The cystic mass was safely resected with laparoscopic surgery without any complication. The final pathological diagnosis was an epithelium-lined duplication cyst in the retroperitoneal space. There was no evidence of malignancy in the duplication cyst. Intracystic bleeding was assumed to be the cause of the abdominal pain. DISCUSSION: The most common differential diagnoses of retroperitoneal cystic masses are pseudocysts related to pancreatitis, cysts from surrounding structures, and neoplasms. In this case, the cystic mass was diagnosed as completely isolated duplication cyst after surgical resection. It is very rarely observed in adults, but it should be listed on differential diagnoses because it has some possibility of malignancy. CONCLUSION: A completely isolated duplication cyst is very rare but noteworthy because there is some possibility of malignancy, ulcerative bleeding, and perforation. A surgical resection is recommended for diagnostic treatment. Laparoscopic surgery is favorable for intraoperative inspection and it is minimally invasive.