We describe an unusual case of primary synovial sarcoma of the kidney. A 47-year-old woman had a tumor massively replacing the right kidney. There were no primary extrarenal neoplastic lesions. Microscopically, the tumor was composed of a cellular proliferation of relatively uniform spindle-shaped cells having atypical spindle or oval nuclei arranged in fascicles with tumor necrosis, without epithelial areas. Immunohistochemically, a small number of the tumor cells were positive for epithelial markers such as cytokeratin and epithelial membrane antigen. The SYT-SSX2 fusion transcripts were detected by a reverse transcription-polymerase chain reaction (RT-PCR) using RNA extracted from formalin-fixed, paraffin-embedded tissue. ETV6-NTRK3 fusion gene transcripts that result from t(12; 15)(p13;q25), which is characteristic of cellular congenital mesoblastic nephroma, were not demonstrated. To our knowledge, this is the ninth case of primary renal synovial sarcoma. This case report indicates that synovial sarcoma should be taken into account for the differential diagnosis of renal spindle cell tumors and the molecular assay detecting the SYT-SSX fusion transcripts is useful for the final diagnosis of synovial sarcoma arising in an unusual location.