The genus Nassella, as currently circumscribed, includes 116–117 American species. It is characterized by florets with a strongly convolute lemma, a conspicuous or inconspicuous crown, and a short palea. Using 53 species of Nassella and 22 outgroup species we conducted phylogenetic analyses to test the monophyly of Nassella and relationships among species. Two plastid (trnT-trnL and rpl32-trnL) and two nuclear ribosomal (ITS and ETS) regions and morphology were used. Our DNA data alone and combined with morphology showed Nassella to be paraphyletic with respect to a monophyletic Amelichloa. Two main clades were recovered: one with species of Nassella distributed in regions of high elevation from Mexico to northwestern Argentina and one composed of the remaining species of Nassella and those of Amelichloa. The latter is mainly concentrated in southern South America in a variety of habitats with generally lower elevation than the other clade. The monophyly of the close relative of Nassella, the South American genus Jarava s. s., was rejected. None of the groups previously circumscribed as subgenera of Stipa, that are now considered to be composed of species in Nassella, were recovered as monophyletic. The close phylogenetic relationship of Nassella and Amelichloa is supported by only one morphological synapomorphy: the lemma margins flat and strongly overlapping.