Synovial sarcoma is a rare malignancy in the thoracic cavity, especially in the mediastinum. In this paper, a case of primary mediastinal synovial sarcoma is reported. A 34-year-old woman was hospitalized with dyspnea. Her chest X-ray and computed tomography (CT) showed a $16{ imes}13{ imes}11$ cm mass in her anterior mediastinal space. Surgical resection was performed but was incomplete. The pathological and immunohistochemical analysis confirmed the diagnosis of monophasic spindle cell synovial sarcoma. The patient underwent adjuvant radiotherapy for two months, but local recurrence and metastasis occurred in her pleural cavity. She eventually underwent chemotherapy for one year and died 18 months after her operation.