A new species of Taosa (Hemiptera: Dictyopharidae) is described, All the stages were collected on the aquatic weed Eichhornia crassipes (Martius) Solms-Laubach (Pontederiaceae) at several localities on the Paraguay River in Argentina, and in the upper Amazon River in Perú. Taosa (Cuernavaca) longula Remes Lenicov sp. nov. is distinguished by the following coloration pattern and morphological features: uniform green coloration with apair of lateral small dark spots on mesonotum, vertex subquadrate with a closed triangular facet well defined on apex, and a long anal segment; in male, the shape and relative length of parameres, the tubular aedeagus with a pair of ventral spinose processes recurved upward; in the female, the length, shape and denticulation of the first valvulae of the ovipositor, To facilitate the comparison of the new species with the morphologically allied, Taosa inexacta (Walker) and Taosa herbida (Walker 1851), a key is included. Further information on reproductive and feeding behavior, host plants, and geographical distribution of this species are given.