Simulium (Simulium) atipornae sp. nov. is described from females, males, pupae, and larvae in Thailand. This new species is characterized in the female by the claw with a small subbasal tooth, ovipositor valve triangular with its inner margin nearly straight; in the male by the style with a short subbasal protuberance and ventral plate Y-shaped, with toothed posterior margin; and in the pupa by the head and thoracic integument almost bare and gill with six filaments. Taxonomic notes are given to compare this new species with nine related species. A new species-group, the christophersi species-group, is proposed to accommodate S. atipornae sp. nov. and nine related species.