Guerramontesia microdonta (Pottiaceae, Bryophyta) is a new genus and species from Argentina and Bolivia. It is distinct from other genera of the Pottiaceae by a combination of characters: absence of central strand in the stem, Ungulate unbordered leaves with yellow KOH colour reaction, costa at midleaf with one band of dorsal stereids, (1)2–3 guide cell layers, absence of hydroids, and sporophyte with peristome of 16 variously cleft, occasionally perforate, coarsely papillose, short and straight teeth. The species is described, illustrated and mapped. Maximum parsimony and Bayesian analyses of cpDNA sequences (trnL-trnF and trnG regions) were undertaken. The resultant trees suggest that the new genus could be one of the earlier diverging lineages within the Pottioideae and Barbuloideae subfamilies of Pottiaceae.