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Euphorbia subg. Athymalus consists of approximately 150 species and is one of the four main lineages that comprise the species-rich genus Euphorbia. Most species in the subgenus are stem succulents with greatly reduced leaves, but there are also leafy herbs, shrubs, trees and geophytes. The subgenus is restricted to arid regions of the Old World. Most species are found in sub-Saharan Africa, with one in Macaronesia and adjacent parts of western Africa, a few in the Arabian Peninsula (one of which extends into Iran) and one native to Madagascar. Twenty-three species are endemic to the northeastern Horn of Africa (SE Ethiopia, Socotra, Somalia), while 72 species are restricted to southern Africa (including Botswana, Lesotho, Namibia, South Africa and Swaziland). Sixty of those are endemic to South Africa alone, where they are particularly associated with the semi-arid west and south of the country in the Greater Cape Floristic Region and the Nama Karoo Region. We sampled 88 species and analyzed data from the nuclear ribosomal ITS and plastid ndhF regions. Analyses of the separate and combined datasets produced phylogenies that confirm the monophyly of E. subg. Athymalus and the placement of E. antso from Madagascar as sister to the rest of the subgenus. Our analyses also show that the subgenus consists of a grade of early-diverging lineages that are relatively poor in species and that the major radiation of succulent species in southern Africa forms a highly supported clade (E. sect. Anthacanthae). Species-level relationships within this southern African clade, however, remain largely unresolved. Our phylogenetic hypotheses allow us to propose a new classification for E. subg. Athymalus where seven sections are recognized, two of which are newly described. The large southern African E. sect. Anthacanthae is further divided into five subsections, and four series are recognized in E. subsect. Florispinae.