The aerophilous diatom Frustulia cirisiae sp. nov. is described from the Subantarctic island Ile de la Possession (Crozet Archipelago). The detailed morphology of this species is examined using scanning electron microscopy (SEM). The taxonomic position of the new taxon is discussed, comparing it with similar freshwater genera such as Frustulia, Amphipleura and Stauroneis. Based on the presence of a stauros, so far an unknown feature within the genus Frustulia, a new subgenus Staurofrustulia is described.