Masticatory muscle myositis is an inflammatory myopathy in which patients most commonly present with jaw pain or inability to open the jaw. This disease is an autoimmune process in which circulating antibodies specifically target the masticatory muscles. Patients can present either in the acute or, more commonly, chronic phase of the disease. Dogs generally demonstrate no other neurologic or physical abnormalities, which may help differentiate this disease from other causes of trismus. Masticatory muscle myositis requires early detection and aggressive immunosuppressive therapy to improve the prognosis.