Solitary fibrous tumor (SFT) of the retroperitoneal space is rare. We report a case of retroperitoneal tumor, diagnosed as SFT. A 69-year-old woman presented with right lower abdominal swelling, and was referred to our hospital with suspicion of right renal tumor. Abdominal ultrasound and computerized tomography (CT) showed a mass (about 15 x 14 x 10 cm) in the right abdomen. The tumor was thought to be right renal rumor, and right radical nephrectomy was performed. In the excised specimen the tumor was not connected to gastrointestinal tract, peritoneum, or right kidney. The histological and immunohistochemical examination of the specimen revealed SFT. The tumor has malignant potential with partially increased mitotic activity and cellularity in the histological examination. The patient is healthy and without evidence of recurrence or metastasis 26 months from surgery.