The genus Megamelus Fieber (Hemiptera: Delphacidae), frequently associated with aquatic plants, includes four species in South America. One of these species, Megamelus scutellaris Berg, was the only species previously recorded in Argentina. In recent explorations in wetlands of South America (northeastern Argentina, eastern Peru, and southeastern Brazil), five species on Pontederiaceae were studied: Megamelus electrae Muir, Megamelus iphigeniae Muir, M. scutellaris, Megamelus timehri Muir, and Megamelus bellicus Remes Lenicov & Sosa sp. nov. Male holotypes of M. electrae, M. iphigeniae, and M. timehri were redescribed; females of both winged forms and brachypterous males of M. electrae and M. iphigeniae, and the female allotype of M. timehri, were described for the first time. M. bellicus is new for science, and it is described based on the following pattern combination: front coloration, anal segment with asymmetrical spine-like processes, shape of pygofer and aedeagus processes in the male, length of the ovipositor, and shape of the valvifer VIII. A key to distinguish the five South American species, based on male and female genitalia and the external morphology, is presented. This work is an update of knowledge on this genus, incorporating new specific diagnostic characters such as the length of ovipositor, shape and position of valvifer VIII, shape and denticulation of the first and second valvulae, and coloration pattern. Further information on host plants, geographical distribution, and biological data are reported.