The species of Diadegma that attack Plutella xylostella (Linnaeus) are revised. Following a morphometric study involving principal components and discriminant analyses, seven distinct morphospecies are recognized. One species is described as new: D. novaezealandiae from New Zealand. Diadegma mollipla (Holmgren) is the name for the species from sub-Saharan Africa and some Indian Ocean and South Atlantic islands. Diadegma varuna Gupta syn. nov. and D. niponica Kusigemati syn. nov. are both synonymized with D. fenestrale (Holmgren). Diadegma xylostellae Kusigemati is strongly presumed to be a synonym of D. semiclausum (Hellén). An illustrated identification key is provided and each species is described in a standard way.