We present a case of gigantic prostate tumor in a patient with castration-resistant prostate cancer with successful local control by external-beam radiation therapy. A 71-year-old man was shown to have a prostate specific antigen (PSA)level of 24.5 ng/ml, Gleason 9, cT2N1M1a, prostate adenocarcinoma with an estimated prostate volume of 26.9 g. He achieved a PSA nadir at 4 months after the initial androgen deprivation therapy and was diagnosed with castration-resistant prostate cancer three years later. Eight months after the diagnosis of castration-resistant prostate cancer, he visited our hospital due to urinary retention. Abdominal computed tomography scan showed a gigantic prostatic mass occupying the whole pelvic cavity along with multiple lymph node, bone and liver metastases. The estimated volume of the prostate was 878 g. A tumor needle biopsy revealed a histological finding similar to the initial prostate biopsy which was adenocarcinoma with Gleason 9. He underwent external beam radiation therapy (60 Gy) to the prostate, which brought about excellent local control with a 96.7% shrinkage of tumor at 2 months after radiation therapy. He had no complaints of urinary symptoms and no need for urethral catheterization until he died of prostate cancer metastases.