A 65-year-old man with a history of combined pelvic external beam radiation therapy (EBRT) and brachytherapy for prostatic adenocarcinoma 15 years prior underwent total pelvic exenteration for presumed rectal sarcoma with prostatic invasion. Pathology revealed carcinosarcoma of prostatic origin. This patient exhibited the longest reported interval between initial presentation with prostatic adenocarcinoma and development of carcinosarcoma. This case is also the first reported case of prostatic carcinosarcoma occurring after combined EBRT and brachytherapy. The increasing use of such combination high-dose radiation therapy may potentially lead to an increased incidence of secondary malignancies such as prostatic carcinosarcoma in the future.