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. andina Franz, 1996, A. maiopensis Franz, 1996, A. pseudangularis Franz, 1996, and A. quinteroi Franz, 1996 belong to different genera and will be treated in forthcoming papers, and one name is placed in synonymy A. picea Raffray, 1904 = A. elfridae Raffray, 1904 syn. nov. The remaining seven species are redescribed, and three new species fitting the concept of the A. cosmoptera group are described A. covidia n. sp., A. pandemica n. sp., and A. quarantena n. sp. The lectotypes of A. cosmoptera (Blanchard, 1851), A. blanchardi Raffray, 1904, A. elfridae Raffray, 1904, A. bifrons Jeannel, 1962, and A. nahuelbutae Franz, 1996 are designated, and the new synonymies A. nahuelbutae Franz, 1996 = A. angularis Jeannel, 1962 syn. nov., and A. caneloi Franz, 1996 = A. temporalis Jeannel, 1962 syn. 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.