Extrapleural solitary fibrous tumors (SFT) and gastrointestinal stromal tumors (GISTs) are similar morphologically and on imaging studies. We report a case of a patient with a giant pelvic tumor, which was difficult to be diagnosed. A 71-year-old male presented to our hospital with a complaint of urinary retention. A radiological examination showed a giant pelvic tumor mass, which ventrally compressed the urinary bladder and rectum, and was supplied by vascular flow from the right internal iliac artery. The resected specimen was 15 × 10 cm, elastic soft, and solid pale yellow in color. Because the tumor was histologically composed of spindle-shaped cells and was CD34 positive, cytokeratin negative, epithelial membrane antigens negative and KIT negative, we first thought the tumor was a KIT-negative GIST. However, additional immunohistochemical staining was DOG1-negative but CD99 and bcl-2 positive. Therefore, we finally diagnosed the patient with SFT. In conclusion, we report a rare case of SFT of the mesorectum to emphasize the importance of bearing this entity in mind and to show that immunohistochemical staining is useful for differentiating this type of tumor from other spindle cell neoplasms, particularly GISTs.