Synovial sarcomas commonly occur in the soft tissue of the extremities, while a primary occurrence in the mediastinum is quite rare. The current study reports the case of an 11-year-old male who presented with a neck mass, which computed tomography showed was due to a giant mediastinal mass involving the thyroid gland. The tumor was resected by thoracotomy and diagnosed as monophasic synovial sarcoma by histopathology. The patient received adjuvant combination chemotherapy and radiation therapy following surgery. At the 3-month follow-up, no local tumor recurrence was found. The present case report highlights the significance of recognizing the unusual presentation and clinical manifestation of synovial sarcoma to aid clinical management. Written informed consent was obtained from the patient's family.