BACKGROUND Primary retroperitoneal mucinous cystadenoma (PRMC) is a rare benign tumor that occurs in the retroperitoneal space. PRMC is difficult to preoperatively diagnose, and final diagnosis requires surgical resection. CASE REPORT A 39-year-old female was referred to our hospital with a chief complaint of tension and pain in the left flank. Imaging findings revealed a large, thick-walled unilocular cystic mass dorsal to the descending colon and ventral to the left kidney. Moreover, a content-rich mural nodule measuring 20 mm was detected on the caudal wall of the cyst, which was suspected to be malignant. The tumor was successfully resected. The cystic wall was composed of thick, scarred fibrous tissue, and the inner surface was lined with a single layer of mucinous epithelium with little atypia. The mural nodule mainly comprised necrotic tissue with no epithelium. Serum carbohydrate antigen 19-9 (CA19-9) level was high (109 U/mL) preoperatively, but it returned to normal levels postoperatively. Importantly, CA19-9 expression was also confirmed using immunohistochemistry. To the best of our knowledge, this is the first case of PRMC with elevated serum CA19-9 levels confirmed by immunohistochemistry. CONCLUSIONS PRMC is only diagnosed by surgical resection. If imaging studies indicate signs of malignancy, surgeons should not hesitate to surgically confirm the diagnosis.