Brachythecium athrocladum (Mitt.) Paris, an endemic species of St Helena Island, South Atlantic Ocean, is re-studied by morphology and molecular markers, nr ITS region and chloroplastic trnL-F. Molecular markers independently indicate the position of the species in Brachytheciaceae subfamily Helicodontoideae, especially close to the genera Helicodontium and Clasmatodon. Common characters of the plants from St Helena with these genera are discussed. Resulting from these studies, the new genus Sainthelenia Ignatov & M. Wigginton is described, with a single species, confined in its distribution to this island.