Sagediopsis bayozturkii, a lichenicolous fungus growing on Antarctic endemic species Acarospora macrocyclos, is described as new to science from three collections made at 65 degrees south in the Argentine Islands (Graham Land, maritime Antarctica). The fungus has a distinct internal apical beak in the ascus and ellipsoid three-septate ascospores; therefore, it is the member of Sagediopsis subgenus Hawksworthiella. Morphologically and anatomically S. bayozturkii is most similar to S. campsteriana and S. pertusariicola, which are confined to the lichen hosts Ochrolechia spp. and Pertusaria spp., respectively. In addition to having different hosts, S. bayozturkii differs from these two species by having smaller ascomata and shorter ascospores. Its similarities to other species of the genus are discussed, and an artificial key based on the published descriptions of the ten species now known in the genus Sagediopsis is presented.