A new species, Plantago rahniana, is described and illustrated. This species has an extremely restricted distribution and is only known from two localities in Santa Catarina, in southern Brazil, which are high-elevation grasslands that are often covered in fog. Based on the IUCN criteria, this species is considered endangered. Material of this species was erroneously identified as Plantago commersoniana, which is also rare and potentially threatened. These two species mainly differ by the shape of their leaves and seeds, and by the pattern of the trichomes on their leaves, bracts, and anterior sepals. The first key to all of the species of Plantago from Brazil is also provided.