Fibroma of the tendon sheath (FTS) involving the temporomandibular joint (TMJ) is very rare. A case of FTS arising from the synovial membrane of the TMJ is presented. The patient was a 17-year-old male with jaw opening pain and clicking in the left TMJ. MRI, arthrotomography, and arthroscopy showed a well-circumscribed mass that was localized in the superior articular cavity of the left TMJ. The tumor was removed with the attached synovial membrane. Macroscopic, histological and immunohistological features of the intra-articular tumor were identical to FTS. There have been no signs of recurrence in the 6 years since surgery.