Schwannomas are tumors originating from Schwann cells in the nerve sheath. Although they can appear on many parts of the body, they occur most frequently in the head and neck. Furthermore, they are very rarely seen in the retroperitoneal region. Ancient schwannoma is a sub-type of schwannomas that is quite rare. In this study, the case of a retroperitoneal ancient schwannoma, which showed cystic degeneration and reached gigantic sizes, is presented. A forty-nine-year-old female patient was admitted to our center with complaints of growing abdominal distention and stiffness for 7 years. In the physical examination, a non-mobile and stiff mass that was approximately 30 cm along its longest axis and extended from the subcostal area to the inguinal region was detected on the right side of the abdomen. A solid retroperitoneal mass containing encapsulated and cystic components was confirmed in the oral and intravenous contrasted abdominopelvic tomography. The size of the mass was 28x20x12 cm, and a total excision of the mass was planned. The mass was excised together with the capsule. In the pathological examination, ancient schwannoma, which showed widespread multifocal cystic degeneration with benign characteristic, was found. In the immunohistochemical staining, S100 was found to be positive. The patient was discharged on the 5th postoperative day. Schwannomas are slowly progressive benign tumors that can reach a large size without any symptoms and rarely settle in the retroperitoneal region. Ancient schwannomas are also a rarely seen sub-type of schwannomas. In this study, the case of a retroperitoneal ancient schwannoma, which showed cystic degeneration and reached gigantic sizes, is presented.