Polygala riograndensis Lüdtke & Miotto (Polygalaceae) is a new species from the Brazilian flora. This species is included in the subgenus Polygala L., which has the largest number of species in the Brazilian territory. Polygala riograndensis is closely related to P. campestris Gardner, but it is set apart mainly by the stems sinuate, capsules winged, leaves petiolate and glabrous, and by the crest of the keel with 6 to 7 lobes that are simple or bifid. This species was recognized during the revision of the genus Polygala in Rio Grande do Sul state. Morphological description, illustration, and observations about habitat and distribution are presented.