This case report describes a novel manifestation of the immune-mediated disease, masticatory muscle myositis. Clinical signs, including difficulty in opening the mouth (trismus), were seen in three of four 12-week-old cavalier King Charles spaniel littermates. Diagnosis was established by 2M immunohistochemistry, supported by characteristic histopathological changes in affected temporal muscle. Treatment using corticosteroids at immune-modifying doses resulted in resolution of clinical signs in all the affected animals. Masticatory muscle myositis should be considered as a differential diagnosis in groups of young dogs with clinical signs of myositis localised to the head.