The present report describes a 31-year-old Japanese woman with a retroperitoneal cystic mass adjoining below the left kidney. No disseminated tumors were observed, and resection of the tumor only was performed laparoscopically. The surgical specimen showed a well-differentiated papillary mucinous cystadenocarcinoma of the ovarian type. Additional gynecological examination, including positron emission tomography-computed tomography, showed no malignancy at other sites. Recurrence or metastasis has not been observed after more than 1 year of follow-up. Primary retroperitoneal mucinous cystadenocarcinoma is a very rare tumor. Thirty-six and 55 cases have been reported previously in the English and Japanese literature, respectively. The pathogenesis of the disease remains unclear and controversial. A standard treatment has not been established in the literature, and a consistent prognosis has not been reported. Therefore, close post-operative follow-up is strongly recommended.