Orthotrichum pilosissimum is described from herbarium material and recently collected specimens in the state of Nevada. Its specific epithet refers to the diagnostic hyper-developed axillary hairs, which are frequently longer than 1 mm, multiseriate and branched, covering most of the stems surface and sometimes protruding from the foliose shoots. These hairs are not only unique in the genus but also among mosses, providing a reliable diagnostic character for the new taxon. Additional gametophytic and sporophytic characters that contribute to its taxonomic delimitation are also given.