Most tumors arising in the retroperitoneum are non-epithelial, and epithelial tumors are very rare. We report a case of mucinous cystadenocarcinoma of the retroperitoneum supposed to be derived from ectopic ovarian tissue. A 56-year-old woman was admitted to the hospital because of abdominal distention, and an abdominal CT scan revealed a cystic tumor of the right lower retroperitoneum. We had been observing the patient for 10 years for this lesion, but at this point, the tumor showed an increase in size and nodules had appeared inside of the cyst. It was presumed to be malignant, and so the decision was made to perform resection. Upon laparotomy, a large cystic tumor was found in the right iliac fossa displacing the ascending colon and the cecum medially. The cyst had no connection with any surrounding structures. The tumor was a simple large cyst, measuring 9×7 cm, with a protruding papillary tumor inside. The inner space of the cyst was filled with mucinous fluid. Histologically, the tumor was diagnosed as a mucinous cystadenocarcinoma. The patient has currently been undergoing follow-up for 7 months with no evidence of recurrence.