During studies toward a new monograph of Promenaea, a new species was discovered in Southeastern Brazil. Promenaea viridiflora is found in the Atlantic Forest, where it is restricted to São Paulo state. The color of the flower, apex of sepals and petals and callus lip are diagnostic characters that distinguish it from P. stapelioides, the morphologically closest species. Nevertheless, Promenaea viridiflora is phylogenetically closer to P. xanthina than P. stapelioides. Although Promenaea viridiflora has been grown successfully in the greenhouse at the Núcleo de Pesquisa Orquidário do Estado in the Instituto de Botânica (IBt—São Paulo, Brazil), it has not been recollected for over 25 years. According to the criteria of the IUCN Red List of endangered plant species, P. viridiflora is assessed as “Critically Endangered”.