SUMMARY: Primarily, innervation of the triceps brachii muscle has been attributed to the axillary nerve. However, clinical reports have observed paralysis from the long head of the triceps brachii muscle following axillary nerve lesions which occurred after dislocation of the glenohumeral joint. This has raised questions about the innervation of the long head of triceps brachii muscle. The objective of this study was to verify the innervation of the long head of the triceps brachii muscle by the axillary nerve. Twelve previously fixed posterior areas of shoulder and arm were dissected and branches of innervation of the axillary nerve towards the long head of triceps brachii muscle were identified. Subsequently, samples were taken for histological hematoxylin-eosin study. In all cases, we observed branches of the axillary nerve penetrating the upper half of the long head of the triceps brachii muscle. The histological study showed an image compatible with nerve tissue in each sample analyzed. The results contrast with the description in classic texts regarding innervation of the triceps brachii muscle, which could present with double innervation from the radial and axillary nerves, or a separate innervation for each head. These results provide information for axillary nerve lesion analysis following shoulder dislocation, at the time of performing surgical procedures in the area, or when planning rehabilitation for these patients.