The genera Nesaea Commers. ex H.B.K. and Ammannia L. in southern Africa, which comprises Namibia, Botswana, South Africa, Swaziland and Lesotho, were revised. In Nesaea, 27 species (seven new), four varieties and one new form were recognized. Ammannia was revised using the DECORANA computer program, and four species were recognized. Keys to the taxa are presented, as well as synonymy (restricted to the region), diagnoses and descriptions o f new taxa, and notes on distribution, taxonomy and nomenclature. The following taxa are new: Nesaea alata Immelman, N. angustifolia Immelman. N. cymosa Immelman, N. minima Immelman. N. sagittifolia (Sond.) Koehne var. ericiformis Koehne forma swaziensis Immelman. N. saluta Immelman, N. wardii Immelman and N. zambatidis Immelman. Keywords: Ammannia, Lythraceae, Nesaea, southern Africa, taxonomy.