The hypothesis, originated by Kohlbrugge (1895) and supported by Lyon (1911), Van Bemmel (1952) and Hill (1960), that there are two taxonomically distinct muntjac (genus Muntiacus Rafinesque, 1815) on Borneo is supported by studies of abundant skin and skull material. The two are widely sympatric, so must rank as distinct species. One represents the widespread M. muntjak (Zimmermann, 1780) of southeast and south Asia, while the other is endemic. The type material of Cervulus pleiharicus Kohlbrugge, 1895 consisted of a skin and frontlet with antlers. As the skin has been lost, the frontlet is designated as lectotype. It represents M. muntjak, as does the type of M. rubidus Lyon, 1911, so that the endemic species of Borneo is without a name. It is hereby designated Muntiacus atherodes. The two species are diagnosed, described fully, and discussed.