Primary retroperitoneal mucinous cystic tumors with borderline malignancy are rarely encountered. To date, only 12 cases have been reported in the literature. In this report, we present an additional case. A 65-year-old nulliparous woman complained of abdominal fullness. Her medical history included a hysterectomy and a single salpingo-oophorectomy performed 25 years prior to the present event. Physical examination revealed a large cystic mass in the abdomen and pelvis. During laparotomy, a cystic tumor measuring 21×14 cm in size was observed in the left retroperitoneal space. The tumor was resected, and the final diagnosis was primary retroperitoneal mucinous cystic cancer with borderline malignancy.