Signet ring cell carcinoma is one of the extremely rare variant of prostatic adenocarcinoma. Only 48 cases have been reported in the literature. The differential diagnosis with metastatic tumors particularly originating from the gastrointestinal system with the same morphological features may be a diagnostic challenge for the pathologist. The presence of conventional adenocarcinoma areas, prostatic acid phosphatase and prostate specific antigen positivity along with cytokeratin 7 and 20 negativitiy are useful criteria for the diagnosis of signet ring cell carcinoma of the prostate. We present a case of primary prostatic signet ring cell carcinoma admitted to hospital with hematuria with the morphological, histochemical and immunohistochemical features of the tumor. This is the second case in medical literature with aneuploidy demonstrated by flow cytometry.