BACKGROUND In recent years, the delayed side effects associated with radiotherapy for prostate cancer have drawn the interest of urologists. Although urosymphyseal fistula is one of these delayed side effects, this serious complication is rarely described in literature and is poorly recognized. METHODS We report our experience in treating a 77-year-old male patient with necrotizing fasciitis after high-dose rate brachytherapy plus external beam radiation for prostate cancer. The patient was referred to our hospital with complaints of inguinal swelling and fever. He had a past history of radiotherapy for prostate cancer and subsequent transurethral operation for a stricture of the urethra. Computed tomography showed extensive gas within the femoral and retroperitoneal tissues and pubic bone fracture. Surgical exploration suggested that necrotizing fasciitis was caused by urosymphyseal fistula. CONCLUSIONS To the best of our knowledge, this is the first case report of necrotizing fasciitis caused by urosymphyseal fistula after radiotherapy for prostate cancer. There is a strong association between urosymphyseal fistula and prostate radiotherapy with subsequent surgical intervention for bladder neck contracture or urethral stricture. Therefore, surgical treatment for bladder neck contracture or urethral stricture after radiotherapy for prostate cancer should be performed with care. The present case emphasizes the importance of early diagnosis of urosymphyseal fistula. Immediate removal of necrotic tissues and subsequent urinary diversion in the present case may have led to good patient outcome.