We encountered a case of primary retroperitoneal mucinous cystadenocarcinoma with ovarian-type stroma, an extremely rare condition. The patient was a 56-year-old woman with complaints of diarrhea, nausea, and abdominal fullness. Abdominal ultrasound revealed a nodular multi-cystic mass in the left lower quadrant of the abdomen. Subsequent contrast-enhanced CT and MRI determined it to be of retroperitoneal origin, consistent with a mucinous cystadenocarcinoma. Numerous small nodules were confirmed during the operation around the tumor suggesting local dissemination of the cancer and a left partial colectomy that included these nodules was performed. On histological examination, mucinous fluid was found trapped inside the cavity and poorly differentiated adenocarcinoma was observed in the solid portions. The ovarian-type stroma was confirmed in the layer beneath the epithelial lining and eventually a final diagnosis of mucinous cystadenocarcinoma was reached.