Penoscrotal extramammary Paget's disease (EMPD) is a rare intraepithelial adenocarcinoma in older male patients, and no effective nonsurgical treatment strategies are currently available. The aim of this study was to report the usefulness of external radiotherapy (RT) for penoscrotal EMPD in an inoperable elderly patient. This report presents the treatment of an 89-year-old man with widespread penoscrotal EMPD. A multidisciplinary treatment team decided on radical RT. The patient received a radiation dose of 61.8 Gy in 30 fractions through electron and photon beams. His treatment tolerance was good, and no severe toxicity had been observed up the last follow-up. At 6.5 years after the RT, the patient showed no evidence of recurrence. Definitive RT resulted in excellent disease control and minimal toxicity; thus, it could be a promising nonsurgical therapeutic option for penoscrotal EMPD, even in extremely elderly individuals.