Placoparmelia patagonica Henssen gen. et sp. nov. is described from two localities in Argentina. The placodioid thallus of P. patagonica has a primitive paraplectenchymatous upper cortex in parts but is devoid of a clearly defined lower cortex, true rhizines or tufts of rhizoidal hyphae. The upper surface is minutely porous and reacts HNO3 + blue-green indicating the presence of glomellifera brown. Norstictic acid with small amounts of connorstictic and cryptostictic acid, and a trac of an unknown substance were detected by TLC. The ontogeny and structure of the apothecia and pycnidia correspond to those of the Parmeliaceae. The anatomy of the thallus in P. patagonica is compared with that in Neofuscelia squamariata, N. pulla and Parmelia saxatilis.