An undescribed marine Pyramimonas, P. australis Andreoli et Moro, sp. nov., forming a bloom in a hole of Terra Nova Bay (Ross Sea, Antarctica) sea ice, was collected, but could not be cultured. Consequently, the description of this new species is based on light and electron microscope observations on samples that were fixed or stored at −80 °C, and its phylogenetic position inferred from nuclear-encoded small-subunit ribosomal DNA (SSU rDNA) and chloroplast-encoded rbcL gene sequences. This is the third Antarctic species described for this genus. The ultrastructure of the cell is consistent with species of the subgenus Trichocystis McFadden, but differs in that it has unique body and cyst scales, and a different encystment procedure. The outermost layer of body scales is formed by flat box scales with peripheral perforations oriented parallel to the four edges and with a further eight central perforations oriented perpendicular to the peripheral ones. Crown scales, which in many other species of the genus form the outermost layer over the entire cell body, were observed in this species in the flagellar pit over the box scales. The flagella are covered by a pentagonal underlayer of scales and by limuloid scales with two subsidiary spines, in addition to the central one. Encystment begins in the flagellate form resulting in a cyst with an irregular wall bearing spine scales. Ultrastructural and molecular data confirm that P. australis belongs to the subgenus Trichocystis.