Otopappus (Compositae, Heliantheae) is revised to contain 15 species from Mexico and Central America. Prominent among the taxonomic difficulties have been generic delimitations, especially with Notoptera. The principal differentia between the two genera, pappus with and without squamellae, is shown not to correlate with other features, and the two genera are merged. Within this complex two sections are recognized: Otopappus (10 species) and Loxosiphon (5 species) with the latter divided into series Loxosiphon (2 species) and series Hirsutus (3 species). Principal characters separating these major groups include discoid vs. radiate heads, numbers of rows of phyllaries and their length in relation to the paleae, color of corollas (white or yellow), and convex vs. conical receptacles. The genus is unified by its viny habit and conspicuously adaxially winged achenes and pappus awns. Within this context, 0. australis, 0. ferrugineus, and 0. simplex seem better referred to Zexmenia and 0. jaliscensis to Lasianthaea. Otopappus seems allied most closely to Salmea or Zexmenia. New combinations are Lasianthaea jaliscensis, Otopappus sect. Loxosiphon, 0. series Hirsutus, 0. guatemalensis, 0. hirsutus, and Zexmenia simplex.