A 3-year-old, spayed female, mixbred dog was referred to the National Taiwan University Veterinary Teaching Hospital with an 8-month history of progressively decreased opening range of the mouth. The dog showed severe bilateral atrophy of the temporalis and masseter muscles. Unilateral mandibular condylectomy and short-term steroid had been given with some benefit. However, symptoms recurred after the steroid therapy was discontinued. Owing to the presence of a high serum antibody titer to type 2M muscle fibers, masticatory muscle myositis was diagnosed. The patient showed an excellent response to the immunosuppressive dosage of prednisolone with symptoms completely resolved. She is currently under long-term follow-up and the given dose of prednisolone is gradually tapering.