A new Thismiaceae species, Thismia prataensis, from the Brazilian Atlantic Rain Forest is described and illustrated. This species was found growing in the “Serra da Prata” mountain range, Morretes, Paraná State, southeastern Brazil, and is distinguished from the other species in the genus by the presence of six bracts in two whorls, free interstaminal lobes, six depressions that surround the throat, double longitudinal lamellae on the outside of the floral tube and stigma with papillose surface. This is the most southerly record for the group in the Neotropical region. A key of the Thismia species inhabiting the Atlantic Rain Forest is also provided.