Primary signet cell carcinoma of the prostate is a rare histological variant of prostate malignancies. It is commonly originated from the stomach, colon, pancreas, and less commonly in the bladder. Prognosis of the classical type is worse than the adenocarcinoma of the prostate. Primary signet cell adenocarcinoma is diagnosed by eliminating the adenocarcinomas of other organs such as gastrointestinal tract organs. In this case report, we present a case with primary signet cell adenocarcinoma of the prostate who received docetaxel chemotherapy because of short prostate specific antigen doubling time.