Among the two most widely distributed duiker species, Philantomba monticola (Thunberg, 1789) and Philantomba maxwelli (C.H. Smith, 1827), the latter shows geographic variation in pelage color and body size. This issue was not investigated in detail so far, especially in the eastern region of its distribution area, notably due to the lack of material from the Dahomey Gap. We undertook a species-level revision of Philantomba in West Africa, notably including a series of specimens collected in Togo, Benin and Nigeria. Using morphological measurements (craniometry) and genetic data (two mitochondrial and three nuclear markers), we describe a new duiker species occurring in the Dahomey Gap (Togo, Benin) and the Niger delta, Philantomba walteri sp. nov. This discovery highlights the importance of the Dahomey Gap for the evolutionary history of the West African forest faunas. It also has conservation implications given that the new species is one of the main targets of the local bushmeat trade.