Background and aims - The African genus Isomacrolobium (Leguminosae, Caesalpinioideae) is poorly known. Its species are revised. Methods - Normal practices of herbarium taxonomy have been applied to study all herbarium material available, mainly at BM, BR, COI, FHO, G, K, LBV, MA, MO, OXF, P, WAG, and YA. Key results - Twelve species are recognised, including one new one: I. brachyrhachis from Gabon. All the species are confined to the Guineo-Congolian region. A full taxonomic treatment with key to the species is given. The new species is fully illustrated. The fruits, known of nine species, are illustrated as well. Distribution maps of all taxa are provided. Macrolobium ernae, a synonym of I. obanense, is neotypified. For I. graciliflorum and I. leptorrhachis a lectotype is designated.