Megalopsallus Knight is revised, with 29 valid species recognized; 8 species are described as new and 13 previously described species are newly treated as junior synonyms. Merinocapsus Knight is treated as a junior synonym of Megalopsallus. The species Europiella albipubscens Knight and Europiella monticola Knight, formerly placed in Megalopsallus are treated as incertae sedis and belonging to Europiella, respectively. Habitus and male genitalic illustrations are provided for all Megalopsallus species; scanning micrographs of the head, scent-gland evaporatory area, vestiture, and pretarsus are included for selected species. A key to males is presented. The majority of Megalopsallus spp. are recorded from dry interior portions of the American West; two species are recorded from the Gulf Coast and East Coast of the United States. Host information is presented for most species, indicating an obligate association with halophytes, most in the families Ephedraceae (Ephedrales) and Chenopodiaceae and Solanaceae (Angiospermae). The relationships of Megalopsallus within the Phylini are discussed.