Primary mucinous cystic cystadenomas of the retroperitoneum are very rarely encountered, and there have been only about 30 cases reported in the literature. The histogenesis of primary mucinous cystadenomas is unclear. Most authors suggested that it develops through mucinous metaplasia in a pre-existing mesothelium-lined cyst. Complete surgical excision is the only treatment and it is required for the final diagnosis and cure. We present here a case report of a 38-year-old Korean woman with primary retroperitoneal cystadenoma. It was a thin-walled, multilocular cyst with a dominant loculus that measured 10.0 x 7.5 x 5.5 cm3 in size, and to the best of our knowledge, this is the first such case to be reported in in Korea.