The species of the genus Ontherus, endemic to Latin America, are revised. A key in English and Spanish for the identification of adults of the 58 recognized species is provided. Three subgenera are recognized (Caelontherus nov., Planontherus nov. and Ontherus s.str.). The following 29 species are described as new: O. hadros, O. magnus, O. gilli, O. ashei, O. politus, O. sanctaemartae, O. tenuistriatus, O. obliquus, O. howdeni, O. diabolicus, O. pilatus, O. sextuberculatus, O. lunicollis, O. monilistriatus, O. pseudodidymus, O. rectus, O. pubens, O. ulcopygus, O. cambeforti, O. raptor, O. planus, O. edentulus, O. rectangulidens, O. lobifrons, O. androgynus, O. amplector, O. atlantidis, O. insolitus, O. stridulator. The following new synonymies are proposed (the first specific name being valid): O. kirschii Harold = O. elongatus Waterhouse; O. alexis (Blanchard) = O. didymus Erichson = O. glaucinus Erichson = O. nevinsoni Waterhouse; O. incisus (Kirsch) = O. thoracicus Waterhouse; O. azteca Harold = O. villosus Luederwaldt = O. strius Howden and Young; O. cephalotes Harold = O. quadrituberculatus Luederwaldt. The variety O. zikani var. carinicollis Luederwaldt is now recognized as a valid species. Species are arranged into groups according to their phylogenetic relationship, with a brief diagnosis given for each of the 11 groups of species. The following information is given for each species, when appropriate: detailed literature review, synonymy, description or redescription, illustration of important morphological characters, data of material studied, and geographic distribution.