Primary synovial sarcoma originating from the cauda equina is extremely rare. Only one case, involving an 11-year-old girl, has been reported. The authors describe the case of a 23-year-old woman with a primary synovial sarcoma of the cauda equina. The patient visited a local hospital and described a 2-month history of low-back pain. She was referred to the authors' hospital for further evaluation. On physical examination, she had a straight-leg raising result of 70° bilaterally. Motor examination revealed Grade 4/5 strength in the bilateral extensor hallux longus muscles. There was normal sensation to light touch and vibration in the lower extremities. Sagittal Gd-enhanced T1-weighted MR imaging demonstrated an intradural, extramedullary, and uniformly enhancing mass that extended from L-3 to L-4. The mass was totally resected and adjuvant local radiation therapy was administered. Reverse transcriptase polymerase chain reaction (RT-PCR) of a paraffin-embedded tissue sample revealed SYT-SSX fusion transcripts, and the diagnosis of synovial sarcoma was confirmed. Five and a half years after surgery, the patient is free of local recurrence and metastatic disease. The RT-PCR detection of SYT-SSX fusion transcripts played a key role in establishing the diagnosis of synovial sarcoma of the cauda equina. Complete resection of the mass with adjuvant local radiation therapy proved to be effective.