Festuca hawaiiensis is a perennial grass with tufted culms, reaching up to 1.5 meters tall. The culms are erect, stout, and glabrous. The leaves are simple and alternate. Blades are 20-30 cm long and 3-5 mm wide, with an attenuate apex that tapers to an involute point. The upper surfaces are scabrous, while the lower surfaces are smooth and often involute at the base. The veins are parallel, and the sheaths are shorter than the internodes. They are glabrous, with a membranous ligule that is 1-2 mm long and has lacerate margins. Stipules are absent. The flowers are arranged in open, erect panicles, 30-40 cm long. The branches are slender, spreading or drooping, 8-15 cm long, branching above the middle, and fasciculate with about five branches in the lower fascicles. The axis is terete (cylindrical), smooth in the lower part but angled and scabrous in the upper part. Each branch bears 3-5 spikelets on pedicels 5-10 mm long, and the rachilla is appressed-hispidulous. The glumes are narrow and glabrous, with scabrous keels near the apex. The first glume is 3-5 mm long and 1-nerved, while the second glume is 6-7 mm long and 3-nerved. The lemmas are narrow, 7-9 mm long and 1-1.5 mm wide, faintly 5-nerved, rounded on the back, scabrous, with an acuminate apex or a short awn of about 1 mm. The palea is as long as the lemma and closely fits its edges, scabrous. The fruit is a reddish-brown caryopsis, oblong to ellipsoid, strongly dorsally compressed, and 4-5 mm long. It grows in dry forests and shrublands, often on old lava flows or in cinder or ash substrates at 2,000 meters.