A new narrow endemic species of Begonia L. (Begoniaceae) from the Atlantic Forest of Brazil is described and illustrated. Begonia caparaoensis E. L. Jacques & L. Kollmann occurs in a small area of Cachoeira Bonita, along the coast of the José Pedro River (20°24′29″S, 41°50′29″W), at the Parque Nacional do Caparaó, Minas Gerais, Brazil, growing on rocky outcrops. Diagnostic characters, description, detailed illustration, and taxonomic comments are given. Begonia caparaoensis resembles B. jureiensis S. J. Gomes da Silva & Mamede, which shares its peltate leaf blades, bilamellate placentae, and obovate anthers, but is easily distinguished by its leaves with serrulate margins (vs. dentate) and its smaller stipules (ca. 7 × 9 mm vs. ca. 3 × 1.5 cm). Begonia caparaoensis is identified as Critically Endangered (CR B2a) according to IUCN Red List criteria.