Primary signet-ring cell carcinoma (SRCC) of the prostate is very rare and has a poor prognosis, even when treated with aggressive therapy. We report herein a case of a 72-year-old man with prostatic SRCC. The patient had a tumor that extended directly to the rectum. Maximal androgen blockade was started and 20 months later, the patient was alive without evidence of recurrence. The present case of prostatic SRCC responded well to medical therapy, however, tumors can recur after a long period of time. Therefore, adjuvant therapy is recommended.