The Achilia cosmoptera species group sensu Jeannel (1962) of the species-rich genus Achilia Reitter, 1890 is revised. Of the twelve taxa placed so far in this informal group, four species – A. andinaFranz, 1996, A. maiopensisFranz, 1996, A. pseudangularisFranz, 1996, and A. quinteroiFranz, 1996 – belong to different genera and will be treated in forthcoming papers, and one name is placed in synonymy – A. piceaRaffray, 1904 = A. elfridaeRaffray, 1904syn. nov. The remaining seven species are redescribed, and three new species fitting the concept of the A. cosmoptera group are described – A. covidian. sp., A. pandemican. sp., and A. quarantenan. sp. The lectotypes of A. cosmoptera (Blanchard, 1851), A. blanchardiRaffray, 1904, A. elfridaeRaffray, 1904, A. bifronsJeannel, 1962, and A. nahuelbutaeFranz, 1996 are designated, and the new synonymies A. nahuelbutaeFranz, 1996 = A. angularisJeannel, 1962syn. nov., and A. caneloiFranz, 1996 = A. temporalisJeannel, 1962syn. nov. are also established. For all these species of the A. cosmoptera group their distribution is detailed and mapped, and habitat/collecting data are summarized.