A new species Allophylus exappendiculatus Somner, Ferrucci & Frazão (Sapindaceae) from a remnant fragment of seasonal semideciduous forest in Rio de Janeiro, Brazil, is here described and illustrated. It can be distinguished from other known species of Allophylus by the simple axillary thyrses closely arranged at the base of young shoots, and by the petals without appendages or rarely with vestigial ones. It is here compared with its putative closest relatives A. edulis and A. puberulus. In addition, the micromorphological characters of the pollen grains are described. Allophylus exappendiculatus is a narrow endemic currently known from six collections from the municipalities of Engenheiro Paulo de Frontin, Itaguaí and Santa Maria Madalena.