BACKGROUND The retroperitoneum is the anatomical compartment positioned behind the peritoneal cavity. It is separated into three primary spaces: the anterior pararenal, perirenal, and posterior pararenal spaces. Retroperitoneal cystic mass is a rare surgical problem that is often wrongly identified before surgery. METHODS We report a case of a 27-year-old female presenting with abdominal swelling and pain starting from 9 months. An abdominal computed tomography scan showed a right adnexal mass with a high probability of a serous ovarian. The patient was diagnosed with ovarian tumors before surgery, but it was identified with a retroperitoneal cyst during surgery. CONCLUSIONS A retroperitoneal cyst's clinical signs and symptoms vary, and the diagnosis can often be challenging. Computed tomography scans are appropriate for assessing retroperitoneal pathology because they produce separate sectional images and couldn't find the correct diagnosis in previous cases. CONCLUSIONS This paper shows the rare case of primary retroperitoneal lesions, which can be hard to diagnose before surgery, even though medical imaging has come a long way.