True metastases to prostate from solid tumors are reported only in 0.2% of all surgical prostatic specimens and 2.9% of all male postmortems. Clinical context, morphological features, and immunohistochemical localization of prostate specific antigen (PSA) are supposed to clarify the differential diagnosis between a secondary and a primary tumor. We report an unusual and rare case of secondary signet ring cell carcinoma (SRCC) of prostate in which the clinical data and signet ring cell morphology pointed toward the diagnosis of a primary SRCC. Immunohistochemistry (IHC) for PSA not only proved the case to be a secondary SRCC but also initiated the process for diagnosis of the occult primary malignancy in the patient's stomach.