The New World assassin bug genus Zelus Fabricius, 1803 (Insecta: Hemiptera: Heteroptera: Reduviidae: Harpactorinae: Harpactorini) is revised based on more than 10,000 specimens. Seventy-one species are recognized and twenty-four described as new: Zelus aithaleossp. n., Zelus amblycephalussp. n., Zelus antiguensissp. n., Zelus auralanussp. n., Zelus bahiaensissp. n., Zelus banksisp. n., Zelus casiisp. n., Zelus championisp. n., Zelus cordazulussp. n., Zelus fuliginatussp. n., Zelus gilboventrissp. n., Zelus gracilipessp. n., Zelus grandoculussp. n., Zelus kartaboidessp. n., Zelus lewisisp. n., Zelus panamensissp. n., Zelus paracephalussp. n., Zelus rosulentussp. n., Zelus russulumussp. n., Zelus spatulosussp. n., Zelus truxalisp. n., Zelus umbraculoidessp. n., Zelus umbraculussp. n., and Zelus xouthossp. n. Five species, Zelus araneiformis Haviland, 1931, Zelus gradarius Bergroth, 1905, Zelus modestus (Stål, 1862), Zelus subfasciatus Stål, 1860 and Zelus vittaticeps Stål, 1866, are removed from Zelus and placed incertae sedis within Harpactorini. Nine new synonyms are recognized (senior synonym in parentheses): Zelus atripes Champion, 1898 syn. nov. (=Zelus conjungens [Stål, 1860]), Zelus dispar Fabricius, 1803 syn. nov. (=Zelus pedestris Fabricius, 1803), Zelus formosus Haviland, 1931 syn. nov. (=Zelus laticornis Herrich-Schaeffer, 1853), Zelus obscuridorsis (Stål, 1860) syn. nov. (=Zelus pedestris), Zelus pallidinervus Haviland, 1931 syn. nov. (=Zelus kartabensis Haviland, 1931), Zelus personatus Berg, 1879 syn. nov. (=Zelus versicolor Herrich-Schaeffer, 1848), Zelus trimaculatus Champion, 1898 syn. nov. (=Zelus means Fabricius, 1803), Zelus trimaculicollis (Stål, 1855) syn. nov. (=Zelus means), and Zelus tristis Haviland, 1931 syn. nov. (=Zelus laticornis). Zelus conjungens (Stål, 1860) stat. rev. Is resurrected from junior synonymy with zealous armillatus (Lepeletier & Seville, 1825). Zelus ambulans Stål, 1862 stat. rev. and Zelus cognatus (Costa, 1862) stat. rev. are resurrected from synonymy with Zelus exsanguis Stål, 1862. Iquitozelus Bérenger syn. nov. is synonymized with Zelus and its only species transferred to Zelus, hence resulting in a new combination, Zelus couturieri (Bérenger, 2003) comb. nov. Lectotypes, paralectotypes or neotypes are designated for a number of species. Habitus images, illustrations of male genitalia, distribution maps and measurements are provided for nearly all species. The three previously recognized subgenera of Zelus are found to be based upon superficial characters and these divisions do not reflect natural groupings. Using sets of characters, especially those of the male genitalia, eleven species groups are proposed. It is also hypothesized that Zelus is closely related to three other New World genera: Atopozelus Elkins, Ischnoclopius Stål and an undescribed genus "Hartzelus" [manuscript name]. Zelus is endemic to the New World, occurring naturally in the Caribbean and all but one of the continental countries, with introductions to Pacific islands, Europe and Chile.