Psoroma saccharatum Scutari and Calvelo (Pannariaceae, lichenized Ascomycotina), from Tierra del Fuego (Argentina), is described as new. It is characterized by the small, areolate, subendolithic thallus extending by means of a stolen-like prothallus, the sugary, fissured upperside, the stratified cortex with well developed epinecral layer, the basal, lateral to overlapping cephalodia, and the rounded spores with roughened epispore, The new species is the host of a parasitic species of Caloplaca Th. Fr.