Primary mucinous cystadenomas of the retroperitoneum is extremely rare, and its histogenesis and biological behavior is unclear. Most authors suggested that it develops via mucinous metaplasia in a pre-existing mesothelium-lined cyst. We report here on a case of a 47-year-old Korean woman with primary retroperitoneal mucinous cystadenoma. Although the tumor was detected by ultrasound and computed tomography (CT), a preoperative diagnosis could not be established. The cystic tumor was successfully removed by laparoscopic surgery and microscopic examination revealed a mucinous cystadenoma. It had an ovarian stromal-like aspect and a lining of monolayer cuboidal epithelium; the tumor measured 5.0×5.0×3.5cm in size.