The large genus Orthomorpha is rediagnosed and is shown to currently comprise 51 identifiable species ranging from northern Myanmar and Thailand in the Northwest to Lombok Island, Indonesia in the Southeast. Of them, 20 species have been revised and/or abundantly illustrated, based on a restudy of mostly type material; further 12 species are described as new: Orthomorpha atypica sp. n., Orthomorpha communis sp. n., Orthomorpha isarankurai sp. n., Orthomorpha picturata sp. n., Orthomorpha similanensis sp. n., Orthomorpha suberecta sp. n., Orthomorpha tuberculifera sp. n., Orthomorpha subtuberculifera sp. n. and Orthomorpha latiterga sp. n., all from Thailand, as well as Orthomorpha elevata sp. n., Orthomorpha spiniformis sp. n. and Orthomorpha subelevata sp. n., from northern Malaysia. The type-species Orthomorpha beaumontii (Le Guillou, 1841) is redescribed in due detail from male material as well, actually being a senior subjective synonym of Orthomorpha spinala (Attems, 1932), syn. n. Two additional new synonymies are proposed: Orthomorpha rotundicollis (Attems, 1937) = Orthomorpha tuberculata (Attems, 1937), syn. n., and Orthomorpha butteli Carl, 1922 = Orthomorpha consocius Chamberlin, 1945, syn. n., the valid names to the left. All species have been keyed and all new and some especially widespread species have been mapped. Further six species, including two revised from type material, are still to be considered dubious, mostly because their paraterga appear to be too narrow to represent Orthomorpha species. A new genus, Orthomorphoides gen. n., diagnosed versus Orthomorpha through only moderately well developed paraterga, coupled with a poorly bi- or trifid gonopod tip, with at least some of its apical prongs being short spines, is erected for two species: Orthomorpha setosus (Attems, 1937), the type-species, which is also revised from type material, and Orthomorpha exaratus (Attems, 1953), both comb. n. ex Orthomorpha .