Securidaca paulensis (Polygalaceae), a new species endemic to the Serra do Mar in São Paulo, Brazil, is described. It is compared to S. lanceolata, from which it differs mainly by its slender branches, leaves with an attenuate apex, racemes predominantly terminal and smaller samaras with non-reticulate seed-bearing core. The study provides a taxonomic description, distribution maps, preliminary conservation status assessment, and an identification key for Securidaca species of the Atlantic Forest.