The brachiocephalic muscle in domestic mammals is formed as a homology of the sternocleidomastoid muscle and the clavicular part of the deltoid muscle since it results from reduction of the clavicle as a clavicular intersection. The cranial insertions of the brachiocephalic muscle vary among species in domestic mammals. In the dog, the brachiocephalic muscle comprises three parts, which arise from the clavicular intersection and insert at the humerus, the dorsal cervical raphe, and the mastoid process of the temporal bone. These three parts are named the cleidobrachial muscle, the cervical part of the cleidocephalic muscle, and the mastoid part of the cleidocephalic muscle, respectively. This complexity could confuse veterinarians and complicate surgical procedures in this area. Information about the normal structure of this muscle, and any variation therein, would help to avoid such situations. During dissections of a male cross-breed dog, we found that the brachiocephalic muscle had two bellies located on the mastoid part of the cleidocephalic muscle that extended from the clavicular intersection to the wing of the atlas and the mastoid process of the temporal bone. They were innervated by the accessory nerve and the ventral branches of the second, third, and fifth cervical nerves, and they were supplied by the ascending branch of the superficial cervical artery. These bellies were considered to be a rare variation of the muscle. This is the second report of a brachiocephalic muscle variation in a dog, in which the mastoid part of the cleidocephalic muscle was made of two bellies inserted independently. Such variations should be considered during anatomical dissections and surgical procedures.