A case of retroperitoneal myolipoma is reported. A 55-year-old woman with the main complaint of an abdominal mass was admitted to Teikyo University Hospital, Tokyo, Japan. Retroperitoneal liposarcoma was suspected based on magnetic resonance imaging, and the tumor was resected. The resected tumor was well encapsulated and 30 x 15 x 8 cm in size. Histologically, it consisted of mature adipose cells and smooth muscle cells. Neither nuclear atypia nor mitosis was observed in either component. The tumor was pathologically diagnosed as myolipoma of the retroperitoneum. Retroperitoneal myolipoma is often misdiagnosed radiologically as liposarcoma because the overwhelming majority of large retroperitoneal tumor containing fat is liposarcoma, however, the clinical course of myolipoma is quite different from that of liposarcoma. Although myolipoma is very rare, pathologists should consider it in the differential diagnosis of fat-containing retroperitoneal masses.