Prostatic stromal sarcoma (PSS) is an unusual lesion that is reported only occasionally. Here we describe a case of prostatic stromal sarcoma in a 33-year-old man who had complained of perineal pain. The serum prostate-specific antigen (PSA) level was above the normal limit at 5.8 ng/ml, and abdominal computed tomography (CT) revealed a giant mass in the retrovesical region. Chest CT demonstrated lung metastases. Specimens obtained by transrectal needle biopsy of the prostate suggested a mesenchymal tumor, but a precise diagnosis required a larger specimen. Palliative transurethral resection (TUR-P) was performed because of obstruction of the urogenital tract, and the final diagnosis was made from this specimen. The tumor contained yellowish gelatinous materials, and the stromal element appeared histologically malignant, with increased cellularity, mitotic figures and pleomorphism. The histological diagnosis was PSS, and the patient received VIP (etoposide, ifosfamide, cisplatin) chemotherapy regimen. However, the pelvic mass continued to increase in size, and the patient's condition rapidly deteriorated and he died. Sarcoma of the prostate gland showing aggressive behavior is quite rare. The detailed histological and immunohistochemical findings in this case are reported, together with a review of the literature.