A primary retroperitoneal mucinous cystadenocarcinoma (PRMC) is an extremely rare lesion. To date, only 49 cases have been reported. The presence of mural nodules in a PRMC may indicate a worse prognosis. We report the case of a 40-year-old Japanese woman with a PRMC with mural nodules. Microscopic examination revealed that the stromal cells of the nodules were spindle-shaped and varied in size. The nodules were immunoreactive for vimentin but negative for cytokeratin and EMA, and the nuclei of the stromal cells were pleomorphic and strongly Ki-67 immunoreactive. The nodules were diagnosed as true sarcoma. To the best of our knowledge, this is 11th published case report of a PRMC with mural nodules.