The new species Santessoniella rosettiformis Henssen is described from a temperate Nothofagus forest in southern Chile and compared with the superficially similar S. pulchella P. M. Jørg. In both species, the thallus is corticate on all surfaces, the characteristic feature separating Santessoniella from Parmeliella. In the blue iodine reaction of the hymenium and the presence of smooth spores, S. rosettiformis corresponds to the closely related S. chilensis (Hue) Henssen whereas S. pulchella with warty spores and reddish brown hymenium in iodine is related to S. polychidioides (Zahlbr.) Henssen, the type species of the genus. A key to the species of Santessoniella is provided; the delimitation of the genus is discussed.