A 19-year-old student who had presented with acute urinary retention was referred to our hospital with biopsy diagnosis of proliferating mesenchymal tumor of the prostate. Magnetic resonance imaging showed enlarged prostatic masses. Suspecting prostate sarcoma, we performed a nerve-sparing radical prostatectomy. Histologic diagnosis of the tumor was prostatic malignant phyllodes tumor. Five months after the operation, bilateral obturator lymph node metastases appeared, which were treated with etoposide, ifosfamide, and cisplatin chemotherapy. After a good response was achieved with four cycles of the chemotherapy, pelvic irradiation was added. Since then, there has been no evidence of recurrence for more than 4 years.