To test the monophyly of the tribe Therini and resolve the relationship between Therini and Cidariini, 49 morphological characters of 38 species of Thera and related genera were analyzed cladistically. Five taxa from the Cidariini were chosen for outgroup comparisons. Monophyly of the Therini was not supported and the Therini newly synonymized with Cidariini. Five genera, Praethera Viidalepp, Thera Stephens, Pennithera Viidalepp, Heterothera Inoue, and Diathera Choi, are redefined monophyletically, and two new monotypic genera are proposed: Costicoma, n. gen. and Fascilunaria, n. gen. The combination Thera firmata (Hübner) is proposed. Relationships among the ingroup taxa are largely resolved: (Fascilunaria(Heterothera(Thera(Diathera, Pennithera)))). Synapomorphies and a diagnosis of each genus are given, and a key to the ingroup genera is provided.