The Antarctic subendemic moss species Didymodon gelidus Cardot has been considered to be identical to the Holarctic D. brachyphyllus (Sull.) R.H.Zander. On the basis of a detailed analysis of the type and very many non-type specimens of the two species, the reinstatement of species rank for D. gelidus is proposed. Diagnostic characters of D. gelidus and its distinction from some closely related species, with which it may be confused, are discussed and illustrated. The species that shares all taxonomically important characters with D. gelidus is the Antarctic endemic Barbula byrdii E.B.Bartram and consequently the names of these two species are considered synonymous, the former having priority.