Abstract The gross anatomy of the palmaris longus muscle in 12 specimens of Saguinus leucopus was studied by dissection. This muscle was constant and unipennate, originated in the medial epicondyle of the humerus; inserted onto flexor retinaculum and palmar aponeurosis; and innerved by the median nerve, and in one case, together with the ulnar nerve. This is the first report of double innervation for this muscle in primates.