We report a rare case of primary signet-ring cell carcinoma of the prostate in an advanced stage. A 62-year-old man with serum level of prostate-specific antigen at 364.70 ng/ml was diagnosed as having cT4N1M1c prostatic signet-ring cell carcinoma of Gleason score 5 + 4 = 9. Immunohistochemical examination demonstrated cytoplasmic immunoreactivity to prostate-specific antigen in signet-ring cancer cells. The intracytoplasmic vacuoles in the signet-ring cells showed mucin production with a positive staining with periodic acid-Schiff. Although the patient received hormonal therapy, the disease progressed and lead to death 15 months after the diagnosis. The clinical and immunohistochemical characteristics of this malignancy are also reviewed.