Africa's montane ecosystems are noteworthy not only for their isolation but for their morphologically similar bird populations that inhabit geographically disparate localities. Many species possess range disjunctions in excess of 2,000 km and appear to represent populations that have been isolated since at least the last Ice Age, including the Northern Double-collared Sunbird (Cinnyris reichenowi). Recent work on other Afromontane birds has demonstrated substantial phylogeographic structure can exist in phenotypically similar populations, with cryptic species occurring parapatrically within the same mountain range. We explored genetic, morphological, and ecological diversity within C. reichenowi to assess whether cryptic regional diversification occurs across the disjunct portions of this species' range. Within C. reichenowi, we find consistent patterns of morphological disparity that coincide with genetic diversification between xeric and wet montane populations within the Cameroon Line in the Western population, and clear genetic differentiation between Western and Eastern populations. Our research demonstrates that the geographically isolated populations of C. reichenowi represent different species, and that ecological diversification is shaping populations within Central Africa. We show here that two named populations should be recognized as members of a western species in the Northern Double-collared Sunbird complex: nominate Cinnyris preussi preussi in the Cameroon Line montane forests, and Cinnyris preussi genderuensis in the more xeric interior of Cameroon and the Central African Republic, likely occurring in adjacent Nigeria as well.