High-throughput sequencing, when combined with taxonomic expertise, is a powerful tool to refine and advance taxonomic classification, including at the species level. In the present work, a new species, Plantago campestris, is described out of the P. commersoniana species complex, based on phylogenomic and morphological evidence. The main morphological characters that distinguish the new species from P. commersoniana are the glabrous posterior sepals and the slightly broader leaves. The new species is known from only three localities, all in natural high-elevation grasslands in Paraná and Santa Catarina states, southern Brazil. According to the IUCN criteria new species should be assessed as Endangered (EN). We present field photographs of P. campestris and related species, and we provide an identification key to the species previously included within the circumscription of P. commersoniana.