Primary retroperitoneal mucinous cystadenomas are rare tumors that are almost always found in women. They are similar to ovarian originated mucinous cystadenoma, but there is no any other evidence of an ovarian origin for primary retroperitoneal mucinous cystadenomas. A 33-year-old woman with complaints of RLQ pain was found to have a cystic mass in the right retroperitoneal space on her abdominal CT scan. The histological diagnosis was confirmed as primary mucinous cystadenoma. We report here on a case of retroperitoneal mucinous cystadenoma, and we also talk about this tumor, including its histogenesis, through a review of the available literature.