Synovial sarcoma is a rare malignant tumor that occurs in the oral and maxillofacial regions, particularly in the mandible. Herein, we report a rare case of biphasic synovial sarcoma of the mandible in an 82-year-old man. The patient was referred with chronic diffuse painful swelling in the right mandible and underwent local resection of the right mandible. Computed tomography revealed severe destruction of the mandible. An initial biopsy revealed osteomyelitis of the jaw. However, immunohistochemical staining and genetic analysis confirmed synovial sarcoma, and the patient underwent additional resection of the mandible. The histological features are described, and the differential diagnoses are discussed.