The world fauna of the genus Nixonia is revised. The genus is described, its relationships with other scelionids are discussed, and a key to world species is presented. Nixonia pretiosa Masner (Namibia) and N. atra Masner (Angola, Democratic Republic of Congo, Kenya, Malawi, Rwanda, Zimbabwe) are redescribed, and 12 new species are described: N. bini, new species (Somalia), N. corrugata, new species (South Africa), N. elongata, new species (South Africa), N. flavocincta, new species (Namibia), N. gigas, new species (South Africa), N. krombeini, new species (India, Laos, Sri Lanka, Thailand, Vietnam), N. lamorali, new species (South Africa), N. pecki, new species (Botswana, South Africa, Zimbabwe), N. priesneri, new species (Egypt), N. sicaria, new species (Namibia), N. stygica, new species (South Africa), and N. watshami, new species (Botswana, Namibia, Zimbabwe). The only host record for the genus is for N. watshami, which been reared from the eggs of Acanthoplus discoidalis (Walker) (Orthoptera: Tettigoniidae, Hetrodinae) in Botswana and Namibia. The phylogenetic relationships among the species are discussed.