In the domestic dog, the medial head (caput mediale) of the triceps brachii muscle arises via a distinct tendon from the crest of the lesser tubercle and the accessory head (caput accessorium) from the proximal caudal part of the neck of the humerus. Here the author presents a previously unreported additional point of origin of the triceps brachii muscle on both antimeres of an adult male cross-breed domestic dog cadaver. The variant head arose via fleshy fibers from the medial aspect of the neck of the humerus, just caudal to the lesser tubercle and fused at insertion with the caudal aspect of the proximal half of the medial head. Thus, it can simply be considered as the second accessory head of the triceps brachii muscle. The radial collateral artery on the left limb passed distolaterally between the medial and variant heads, whereas it passed caudal to the variant head on the right limb. The communicating branch between the musculocutaneous and median nerves extended in both directions on the left brachium. Besides a potential clinical significance, knowledge of this variant can be useful in understanding the embryogenic and phylogenetic perspectives.