The species of the genus Monomachus Klug occurring in the Americas are revised. The genus is found from Chile and Argentina north to the Mexican states of Oaxaca and Chiapas. A total of 20 species are considered to be valid and 9 are described as new: M. atratussp. nov., M. aurifersp. nov., M. cultratussp. nov., M. exulsp. nov., M. intonsussp. nov., M. paulussp. nov., M. satyrussp. nov., M. serratussp. nov., and M. velatussp. nov. The species M. bicolor Szépligeti, M. cubiceps Schrottky, M. eurycephalus Schletterer, M. fuscator (Perty), M. glaberrimus Schletterer, M. klugii Westwood, M. megacephalus Schletterer, M. pallescens Schletterer, M. porteri Brèthes, M. segmentator Westwood, and M. variegatus Schletterer are redescribed. Monomachus lateralis Westwood is considered a nomen dubium stat. nov.; a lectotype is designated for M. cubiceps; and Monomachus fuscator andinus Schulz is treated as a junior synonym of M. fuscator, syn. nov. A key to the species of the New World and a checklist of the world species is provided. Monomachus fuscator and M. eurycephalus are parasitoids of the coffee pest Chiromyza vittata Wiedemann (Diptera: Stratiomyidae, Chiromyzinae).