Dystonia affecting the perioral and pharyngeal regions is called oromandibular dystonia (OMD). OMD can affect the jaw muscles and may cause trismus, mouth-closing disturbance, tooth wear/loss, and damage to the oral soft tissues and temporomandibular joint (TMJ). This case history reports a case of jaw-closing OMD accompanied by trismus-related TMJ osteoarthritis (OA). In the present case history, treatment for OMD was started as soon as the diagnosis of jaw-closing OMD was established, and dental treatment intervention for preventing its development or progression was considered at the same time.