The traditional circumscription of the genus Stonesia G. Taylor (Podostemaceae, Podostemoideae) includes three species restricted to western tropical Africa. Here, a new species, S. ghoguei E. Pfeifer & Rutishauser, is described, which represents the first Cameroonian member of the genus. There are another three Stonesia species restricted to western tropical Africa (Guinea and Sierra Leone). The genus Stonesia is characterized by capsule valves with five or seven ribs each, with the ribs nearest the sutures shorter and not reaching the ends of valves. This unique pattern is shared with the two Madagascan genera Endocaulos C. Cusset and Paleodicraeia C. Cusset. Molecular (matK) data indicate that this capsule pattern is homoplastic, occurring in Madagascar and (with Stonesia) in tropical Africa. Various characters of Stonesia (including S. ghoguei) are also found in other African podostemoids. These include flower buds inverted in the sac-like spathella; crustose roots or broad ribbons with exogenous root lobes (daughter roots); stems usually simple (rarely branched), up to 10(–40) cm long; flowers arising primarily from endogenous buds inside the stem cortex in S. ghoguei and S. fascicularis G. Taylor; and leaves repeatedly forked into narrow segments, with epiphyllous flowers arising from the clefts of these forks in S. ghoguei and S. heterospathella G. Taylor. Unlike the western African Stonesia species, S. ghoguei has pollen mainly released in monads (not only dyads), one stamen per flower with two lateral tepals (not two stamens with three tepals), and unilocular ovaries (not bilocular ones).