A 39-year-old woman presented at our hospital with abdominal distension and left lumbalgia. A computed tomography scan revealed a large, heterogeneous mass with a combination of cystic, fatty, and calcified components. The CA125 levels were elevated. Initially, we suspected a mature ovarian teratoma in this case. However, upon reviewing the CT findings, it appeared that the tumor was in contact with the left iliopsoas muscle and the left iliac artery, suggesting a retroperitoneal tumor. Our patient underwent reductive surgery, and the pathological examination confirmed a dedifferentiated retroperitoneal liposarcoma.