Abstract
A new species of Bertolonia (Melastomataceae; Bertolonieae) endemic to the Brazilian Atlantic Forest is described and illustrated. Bertolonia organensis is known from only one locality in the Serra dos Órgãos National Park, state of Rio de Janeiro. The main diagnostic characteristics that distinguish B. organensis are the leaves with bullate adaxial surface and foveolate abaxial surface, cordate base and seven acrodromous veins, petiole and hypanthium glandulose-punctate, setulose and setulose-glandulose, the external calyx lobes erect, thick, narrow-triangular, apex acuminate-glandulose, margin entire, not ciliate, and the anthers connective dorsally appendaged, trilobed or with an acute calcar. Comparisons with similar species, geographic distribution and habitat are presented for the new species, as well as a key to identify all Bertolonia taxa known from the state of Rio de Janeiro. Bertolonia organensis is classified as Critically Endangered (CR).