Mucinous cystadenomas and carcinomas of the ovary are well-established and common tumors. However, Primary retroperitoneal mucinous cystadenomas (PRMCs) are very rare. We present a 50-year-old woman referred to our institution with history of intermittent abdominal pain and palpable mass in her left flank. On CT-scan an unilocular cystic mass with thin wall measuring 171×155×108mm in the retroperitoneum was shown. Patient underwent surgical excision of the tumor. The histopathology examination showed a cystic mass with a fibrous wall and an epithelium composed of a single layer of columnar cells with mucin vacuoles compatible with PRMC.