Geodiscelis thaumaskelos Packer, new species, is described from Mendoza Province, Argentina. The species is unique among all known Xeromelissinae in having only one metatibial spur in both sexes and the almost glabrous second sternum in females, which in other Xeromelissinae bears a scopa that is usually better developed than that on the hind leg. The males are unique among all bees in the enormously expanded metabasitarsus that is deeply concave on the outer surface and which bears a complex pattern of hair brushes. Like the other two species of the genus, G. thaumaskelos forages on Boraginaceae.