We report a case of pyoderma gangrenosum of the penis presenting as Fournier's gangrene. A 77-year-old man who had undergone radiotherapy for localized prostate cancer 16 month earlier, presented with penile pain and fever. Symptoms began with erythema and induration on the dorsal surface of the penile shaft followed by spontaneous purulent drainage with severe pain. Magnetic resonance imaging was unremarkable except for swelling of the penile skin. Biopsy of the ulcerative penile lesion demonstrated a nonspecific inflammation without vasculitis or malignancy. Despite broad-spectrum antibiotics and debridement, the penile lesion extended and new satellite lesions developed as pustules on the glans. Since cultures were negative for aerobic and anaerobic bacteria, a course of intravenous prednisolone was then initiated at 100 mg/day. Within 24 hours the temperature normalized, progression of the penile lesions stopped and became convalescent. The steroid was then tapered and discontinued. The penile lesions healed slowly during the subsequent 1-month period. Based on the clinical course and histopathological findings as well as exclusion of other ulcerative conditions, a diagnosis of pyoderma gangrenosum was made. Penile involvement of this non-infectious ulcerating skin disease has rarely been reported. Pyoderma gangrenosum affecting the penile skin, such as that in present case, may show a similar presentation as Fournier's gangrene. Prompt differential diagnosis is mandatory since effective management for each processes is markedly different.