Synovial sarcoma is a malignant neoplasm predominantly affecting the soft tissues of the extremities of adolescents and young adults. Primary mediastinal synovial sarcomas are rare and have been recognized only recently. We report a case of this rare disease with the initial presentation of back pain. Chest plain film and computerized tomography revealed a huge right-side tumor mass with pleural effusion. Computerized tomography-guided biopsy disclosed synovial sarcoma. A general survey showed no evidence of distal metastasis. The patient received palliative debulking surgery, had a poor response to adjuvant radiotherapy and chemotherapy, and expired 6 months after operation. Due to the rarity of primary mediastinal synovial sarcoma, there is no standardized therapy. Broad surgical resection is the cornerstone of therapy. We report a patient with this rare disease and review the literature.