Two cases of primary prostatic synovial sarcoma presenting as a prostatic mass are presented in patients aged 44 and 46 years. Histologically, both tumors were mainly composed of uniform spindle cells forming interlacing fascicles. Clusters of immature epithelioid cells were also observed among the spindle cells in case 1. Immunohistochemically, the tumor cells of both cases were strongly positive for vimentin, bcl-2, CD99, and E-cadherin, as well as focally positive for cytokeratin. However, they were negative for prostate-specific antigen, S-100 protein, CD34, CD117, muscle-specific actin, desmin, and calretinin. The presence of an SYT-SSX gene fusion resulting from t(X;18) was demonstrated from paraffin blocks by reverse transcriptase polymerase chain reaction in both cases. To the authors' knowledge, these represent the fifth and sixth reported cases of prostatic synovial sarcoma. Accurate diagnosis depends on morphologic and immunohistochemical examination and proper molecular analysis.