(1) Background: Trismus clinically manifests as a reduction of the buccal opening and restricted mouth opening due to different etiologies, but it is often associated with traumatic phenomena. Several treatments have been proposed such as physiotherapy exercises, cryotherapy, laser therapy, hyaluronic acid and platelet-rich fibrin infiltration, but the gold standard is represented by drug therapy based on corticosteroids and NSAIDs, currently the most documented in the scientific literature. Capacitive-resistive electric transfer (Cret) therapy is used to treat musculoskeletal injuries. Cret is a non-invasive electrothermal treatment classified as deep thermo-therapy. (2) Patient: We would like to document a case of particularly traumatic trismus in a 12-year-old patient, not responsive to previous pharmacological therapy and treated with a radiofrequency device called Velvet temporomandibular joint (TMJ). Five capacitive and resistive diathermy sessions with the device were performed. The first four sessions were performed every 4 days and the fifth after 5 days. (3) Result: The maximum opening of the mouth was 10 mm at the initial stage and 38 mm at the end of the six sessions. Pain regressed after the second appointment. (4) Conclusions: Clinical studies with a good number of samples need to be conducted to evaluate the effectiveness of this device which has proved to be an excellent treatment for this refractory case to conventional therapies. Finally, it may be useful to define precise and replicable protocols to make this therapy suitable for patients with TMJ disorders.