We report a case of biphasic synovial sarcoma of the mediastinum, a very rare tumor, in a 12-year-old boy with left-sided chest pain of 3 years duration at presentation. Chest X-ray showed left-sided opacity with loss of cardiac silhouette and the mediastinum deviated to the opposite side. Computed tomography (CT) of thorax showed left-sided posterior mediastinal mass with left-sided pleural effusion and pleural thickening. CT guided fine needle aspiration cytology (FNAC) from the mass reported it as spindle cell variant of adenocarcinoma. Ultrasonography (USG) of the whole abdomen revealed no abnormality. The mediastinal tumor was resected by left thoracotomy and histopathological report confirmed it to be a biphasic synovial sarcoma with capsule invasion at places.