The Southern Hemisphere genus Psoromidium Stirton (Lecanorales, Pannariaceae) is closely related to certain elements within the heterogeneous genus Psoroma Michx, but is distinguished by the absence of a photobiontin the apothecial margin. In Psoromidium, as in Psoroma, the main photobiont is green and is present in a layer below the upper cortex of the squamules; Nostoc occurs in scattered, well-defined cephalodia. Two species are known: P. aleuroides, with an Australasian distribution, and P. versicolor, anaustral species from Tasmania, Fuegia and the subantarctic Auckland Islands group of New Zealand. Anatomy, morphology, distribution, habitat ecology and taxonomy of the two species are discussed. Psoromaria Nyl. is a synonym of Psoromidium.