Primary retroperitoneal mucinous cystadenocarcinoma is a rare tumor. Only about 30 such cases have been reported in the worldwide literature, and a few Korean cases have been reported. The pathogenesis is not clear, and coelomic metaplasia of the retroperitoneal mesothelium has gained wide support. There is no consensus on the appropriate treatment, but surgical exploration is needed for the diagnosis and treatment, and adjuvant chemotherapy may be recommended following complete surgical excision. The long-term prognosis has not been established. We report here on a 32-year-old woman who was diagnosed as having a retroperitoneal mucinous cystadenocarcinoma with mural nodules of sarcomatoid change. Tumor excision and adjuvant chemotherapy were done and the patient is doing well without any evidence of recurrence at 42 months postoperatively.