Mucinous neoplasms are commonly seen in the ovaries and pancreas. Their occurrence in the retroperitoneum is uncommon. We present a case of a retroperitoneal mucinous cystadenocarcinoma in a 54-year-old female who presented with right flank pain. Imaging demonstrated an 8.6 × 7.9 cm mass at the anterior surface of the lower pole of the right kidney, suspicious for renal cell carcinoma. Serum tumor markers carbohydrate antigen 19-9 (CA 19-9) and cancer embryonic antigen (CEA) were within normal limits, and cancer antigen 125 (CA 125) was elevated. Surgical resection of the mass was performed. Intraoperatively, the mass was noted to lie in the retroperitoneum, unattached to the kidney. On gross examination, a 10.0 × 7.0 × 7.0 cm unilocular cystic structure with red-brown mucoid material was present. The inner lining was mostly smooth with areas of excrescences, covering less than 5% of the surface area. Microscopic examination showed cystic areas lined by mucinous epithelium with an underlying ovarian-type stroma. Solid areas showed features of a borderline papillary mucinous tumor with invasive carcinoma. A diagnosis of mucinous cystadenocarcinoma was made. Their occurrence in the retroperitoneum is unusual. Although rare, this entity should always be considered in the differential diagnosis of retroperitoneal cystic lesions.