A new species of Persicaria, P. humboldtiana, endemic to a narrow area of waterfalls in Corupá, southern Brazil, is described in the present paper. The locus classicus of the new species is well-known for plant endemisms. A complete morphological description, original pictures, a distribution map, and a morphological comparison with the similar Eurasian species P. minor (≡ Polygonum minus) are given. The name Polygonum minus is lecto- and epitypified on, respectively, a Morison’s illustration and a specimen preserved at BM.