Cecidiomyces santessonii gen. & sp. nov., found on Stereocaulon species on the subantarctic Macquarie Island (Tasmania, Australia), is described, illustrated and compared with other morphologically similar genera and species. It is characterized by a lichenicolous habit, gall-formation, caespitose to sporodochial colonies, semi-macronematous, monoblastic, determinate conidiophores reduced to conidiogenous cells, solitary, 0–1-euseptate, pigmented conidia, and schizolytic conidial secession. anamorphic fungus, gall formation, lichen-inhabiting, new taxa, Stereocaulon