Primary retroperitoneal mucinous cystadenocarcinoma is a very rare malignancy, and little is known concerning its pathogenesis, optimal treatment, and prognosis. A 29-year-old pregnant woman (21 weeks) presented with abdominal discomfort. CA 19-9, CA 125, and CEA were normal. Abdominal CT scanning revealed a $19{ imes}15{ imes}13cm$ retroperitoneal tumor. Exploratory laparotomy and tumor excision were performed. Mucinous retroperitoneal implants were removed as completely as possible. Histologically, the tumor showed focal areas of capsular invasion, but free resection margins. The uterus and both ovaries were normal in appearance. No adjuvant therapy was pursued. Six months later, peritoneal and bilateral ovarian metastases were discovered. Hence, we report the details of this case of primary retroperitoneal mucinous cystadenocarcinoma and present a review of the literature.