Synovial sarcoma is an extremely rare tumor of the mediastinum. Although it occurs predominantly in the soft tissues of the extremities, this neoplasm has been described in a wide variety of other locations. We report a case of synovial sarcoma occurring in the mediastinum. This 24-year-old male patient presented with dry cough and chest pain. Imaging studies revealed a huge mass lesion originating from the right lower mediastinum with right lower lung invasion and right rib destruction. Computed tomography-guided biopsy confirmed the diagnosis of synovial sarcoma. He underwent surgery and post-operative irradiation therapy. His condition was stable 1 month after the operation.