A 10-yr-old male, neutered gray wolf (Canis lupus) was presented for atrophy of the temporalis and masseter muscles. Clinical signs and magnetic resonance imaging were consistent with a myopathy. Positive serology for antibody titers directed against Type 2M myofibers, and the observation of a mixed mononuclear inflammatory cell infiltrate along with eosinophils and neutrophils within the temporalis muscle, were diagnostic for masticatory muscle myositis. Importantly, protozoal myositis was excluded based on other clinicopathologic data. The case highlights the potential for immune-mediated polymyositis in canids other than the domesticated dog (Canis lupus familaris). Additionally, awareness of a diet in which raw meat is used should prompt a thorough investigation for an underlying infectious myositis in the gray wolf.