Monteiroa rubra is described as a remarkable new species from Rio Grande do Sul, Brazil. It is notably distinct from the other ten species of Monteiroa. The new species is morphologically most similar to the four species of the genus with unlobed leaves that also lack cordate leaf bases. Monteiroa rubra apparently is an endemic species known from three localities in wetlands associated with grasslands in Augusto Pestana and Fontoura Xavier municipalities in southern Brazil. A morphological description, distribution map, photographs, ecological notes, and a key to morphologically most similar species are presented. According to IUCN criteria, if a formal assessment were performed, the new species would probably be considered Endangered.