In this treatment we revise the circumaustral fern species of Megalastrum found primarily between 33–45° latitude South. These species are found in the Valdivian temperate rain forests of Chile and Argentina, the Juan Fernández Islands in the South Pacific, the Tristan da Cunha archipelago in the southern Atlantic, and Île Amsterdam and Île Saint-Paul in the southern Indian Ocean. Megalastrum is absent from Australia and New Zealand. Seven species are treated in total. Two species and two varieties of Megalastrum have generally been recognized in Chile and the Juan Fernández Islands. In this treatment, we recognize four species for the region, one of which is elevated to the species rank (M. glabrius stat. nov.), and one which is new (M. masafuerae sp. nov.). Within the Tristan da Cunha archipelago, two species occur: M. aquilinum comb, nov., and another here described as new, M. peregrinum sp. nov. Île Amsterdam and Île Saint-Paul harbor one species that is described here as new: M. taafense sp. nov. For each species we provide a key, descriptions, complete synonymy, discussions, specimens examined, and illustrations.