Abstract
Begonia margaretiana, a new species of Begonia sect. Pritzelia from Rio de Janeiro state, Brazil is described. Begonia margaretiana is morphologically similar to Begonia paulensis, with which it is compared. The new species grow in hygrophilous Atlantic Forest in the south of Rio de Janeiro state, Brazil. Description, comparisons with a morphologically similar species, etymology, taxonomic comments, illustrations, a map, and the conservation status of the new species are provided.