We report an unusual case of a large solitary fibrous tumor (SFT) in the retroperitoneum. A 53-year-old man was referred to our hospital for surgical treatment of a swelling in the right flank with dull pain. Abdominal computed tomography (CT) and echograms showed a large encapsulated tumor compressing the right kidney and liver. At laparotomy, the tumor was found to be encapsulated but fixed to the capsule of the right kidney within a small area. Therefore, complete removal was achieved. The resected specimen was an encapsulated elastic hard tumor, 14 x 13 x 10 cm in size. Immunohistochemical studies revealed reactivity for CD34 and vimentin, but no staining for keratin, S-100, or alpha-smooth muscle actin, confirming a diagnosis of SFT. Although SFT is usually associated with a favorable prognosis, close follow-up is recommended because of the limited information on its long-term behavior.