We present a detailed morphological study of three South American species of Ormosia (Leguminosae, Papilionoideae, genistoid clade) that have been confused in herbaria and in the literature. The three species are morphologically linked by the shared possession of relatively few leaflets, small, thin-walled fruits, and relatively small seeds, and they each occur in habitats associated with white sand deposits. Individually diagnostic morphology and non-overlapping geographical distributions support their specific distinction from each other and from other closely related species of the genus. Ormosia costulata is recircumscribed to include only populations in Suriname and French Guiana, while Ormosia trifoliolata, previously treated as a synonym of O. costulata, is lectotypified and re-established as a distinct species ranging across much of the Brazilian Amazon and western Guiana Shield. Material formerly identified as O. costulata from the Atlantic Forest domain in the Brazilian state of Bahia is recognized as a distinct and endangered species, herein described and illustrated as O. lewisii.