Background Retroperitoneal cysts (RPCs) are uncommon with an estimated incidence of 1/5750 to 1/250,000. In this entity, non-pancreatic pseudocyst are the rarest with only handful of cases reported in literature, out of which all were found in males. Most of the time they are discovered incidentally and diagnosis depends in part histologically on absence of an epithelial lining to the cyst wall, and the patient’s history represents the cornerstone for diagnosis prediction. Case presentationA 27 year old, female patient presented to the Gynaecology out patient department with complaints of amenorrhea for 2 months & lower abdominal discomfort. On investigation, there was an incidental finding of a large multi-cystic lesion arising from the pelvis on transvaginal ultrasound scan. The patient was then referred to the General Surgery department for further management. Correlation of the clinical scenario, imaging and post- operative histopathological examination of the specimen, revealed Retroperitoneal Non-pancreatic Pseudocyst. ConclusionRPC’s are very rare, and probably remain quiescent, until attain considerable size. Usually, symptoms are non-specific and on examination there may be a palpable, freely mobile abdominal mass. Sometimes, subjected to one of its classical complications such as infection, rupture or haemorrhage enforcing the patient to seek urgent medical advice. In view of potential development of symptoms and complications, the treatment protocol lines with complete excision of the cyst in order to prevent any recurrence