Mosquito species of the Anopheles nili group (Diptera: Culicidae) transmit malaria to humans along rivers in Africa. To date, the An. nili group includes the species Anopheles nili s.s. and its pale-winged variant known as the "Congo form," Anopheles somalicus and Anopheles carnevalei. Larval and adult mosquito collections in the forest region of Campo, in southern Cameroon, uncovered an additional morphological variant provisionally called "Oveng form" that was subsequently found to be genetically distinct from the other members of the An. nili group. In this study, we provide further biological data that characterizes this new taxon and justifies elevation to specific rank. We propose calling this new species Anopheles ovengensis, after its geographical origin. We present a morphological description of the adult female and fourth instars and original data on the biology, ecology, and role as a human malaria vector of this new species in its type location. We provide dichotomous keys for identification of adult females and fourth instars that can be used at least in tropical areas of west and central Africa.