Two new species of Gyrodactylus Nordmann, 1832 (Gyrodactylidae, Monogenoidea) are described, i.e., Gyrodactylus coriicepsi n. sp. from gills of black rockcod, Notothenia coriiceps Richardson, 1844, and Gyrodactylus nudifronsi n. sp. from gills of gaudy notothen, Lepidonotothen nudifrons Lönnberg, 1905, collected in waters surrounding the South Shetland Islands, Antarctica. Gyrodactylus coriicepsi n. sp. is characterized by a long, narrowing membrane of the ventral bar and a marginal hook sickle with beveled heel and toe. Gyrodactylus nudifronsi n. sp. is characterized by a conspicuous ventral bar process and barely curved marginal hook sickle with curved heel and toe. Definitive species identification and phylogenetic position were based on sequences of the rDNA region (ITS1-5.8S-ITS2). Both new species belong to a monophyletic group (bootstrap support 99%) that includes the European marine species Gyrodactylus perlucidus Bykhovskiy and Polyanskiy, 1953 (on Zoarces viviparus), Gyrodactylus flesi Malmberg, 1957 and G. robustus Malmberg, 1957 (both on Platichthys flesus), and the freshwater species Gyrodactylus hrabei Ergens, 1957 and Gyrodactylus mariannae (both on Cottus poecilopus). The observations suggest an evolutionary continuum of the marine Gyrodactylus spp. fauna in the northern and southern hemispheres, which indicates the marine origin of some freshwater Gyrodactylus species.