Nimisia fuegiae Kärnef. & Thell gen. et sp. nov. is described from Argentinian Tierra del Fuego. The new lichen differs from possibly related groups within the Parmeliaceae in unique anatomical characters, i.e. a medullary layer composed of extremely thick and strongly gelatinized periclinally arranged hyphae and a lower cortex composed of only a few layers of strongly pigmented, rather large cells. Ascus characters support its position in the Parmeliaceae. The new genus, which is named in the honour of its first discoverer, is discussed in relation to several other well-investigated members of the family.