Extraovarian primary retroperitoneal mucinous cystadenocarcinoma is an exceptionally rare malignant tumor. We report a case of a 71-year-old Caucaisan female with 2 week history of abdominal tenderness and mild pain in lower abdomen. Radiographic imaging showed an enormous cystic tumor of unknown origin in the left retroperitoneum. The patient was prepared for surgical removal of the cystic growth, presumably an ovarian tumor. Laparotomy revealed a large cystic tumor covered with omentum. The cyst was punctured, thus facilitating a complete removal of the tumor. Adjuvant hysterectomy with bilateral salpingo-oophorectomy was performed, due to growth’s proximity to the left ovary. Histological examination unveiled that the removed tissue is an extraovarian retroperitoneal primary mucinous multicystic adenocarcinoma. We present this case since primary retroperitoneal cystadenocarcinomas are diagnostically challenging and there is no available evidence based guidelines concerning treatment, especially neoadjuvant chemotherapy and adjuvant hysterectomy seem to be controversial.