Sphaerellothecium siphulae growing on Siphula ceratites is described as new from the Russian Arctic and compared with similar species of the genus. The species is distinct by its crowded ascomata, only sometimes associated with a macroscopically visible net of dark hyphae, the absence of interascal filaments, the host genus and a strong pathogenicity. Keywords: Ascomycota; lichen parasites; new species; Russia.