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Icefree, terrestrial, Antarctic ecosystems have a polar desert-like appearance with a scarce vegetation cover, completely dominated by bryophytes and lichens. Knowledge of the rich Antarctic lichen biodiversity, including c. 400 species, is therefore necessary, also for studies of other ecosystem components. The genus Psoroma is partly dominant there, and own ongoing research indicates that many of its members have been misunderstood. The aim of the present study is to describe Psoroma antarcticum as new to science, study its habitat ecology and total distribution, and include a genetic analysis with respect to its internal variation and relationship to other species. The species differs from the closely related species Psoroma hypnorum, in having distinctly cup-shaped apothecia with thalline excipuli without squamules and tomentose lower sides, in having shorter ascospores without apical nodulose extensions and thalli with grey-to-black melanins. It is shown to form a monophyletic clade based on an analysis of the ITS, LSU, and mtSSU loci, and this clade is included in the Psoroma s. str. clade, which includes P. hypnorum, Psoroma paleaceum, Psoroma buchananii, and Psoroma fruticulosum with high statistical support. The new species has its main distribution in the maritime South Shetland and South Orkney Islands of Antarctica, and most samples originate from King George Island, where it is common and an important component in polar desert-like vegetation. Scattered occurrences have also been found in Chilean Tierra del Fuego, South Georgia, and Bouvet Island.