Generic delimitations and understanding of relationships among the herbaceous Afro-Asian genera Ammannia L., Hionanthera A. Fern. & Diniz, and Nesaea Comm. ex Kunth were long elusive due to the extensive similarity in their habit and floral morphology. Recently, studies based on morphological and molecularbased evidence found the three genera forming a strongly supported monophyletic clade. The need for taxonomic unification as a result of this finding is satisfied here by formal nomenclatural recognition of a single genus under the earliest available name, Ammannia, and new synonyms Hionanthera and Nesaea. To accommodate the required nomenclatural changes, 77 new combinations are proposed. Seven new names are provided for replaced synonyms: Ammannia brevistyla S.A.Graham & Gandhi, A. fernandesiana S.A. Graham & Gandhi, A. grayi S.A. Graham & Gandhi, A. involucrata S.A. Graham & Gandhi, A. mauritiana S.A.Graham & Gandhi, A. parvula S.A. Graham & Gandhi, and A. pauciramosa S.A. Graham & Gandhi. Lectotypes are designated for 18 names.