Plants perennial; cespitose. Culms 40-100 cm. Sheaths villous distally, margins ciliate; ligules about 1 mm, densely ciliate, hairs white; blades 15-30 cm long, 5-8 mm wide, both surfaces villous. Panicles 10-20 cm, loosely spicate; bristles usually solitary, 10-20 mm. Spikelets 2.8-3 mm. Lower glumes about 1/3 as long as the spikelets, broadly ovate, 3-veined; upper glumes nearly equaling the spikelets, 5-7-veined; lower lemmas equaling the upper lemmas, 5-veined; lower paleas about 1/5 as long as the upper paleas, lanceolate; upper lemmas finely and transversely undulate-rugose basally, striate and punctate distally; upper paleas similar, ovate-lanceolate. 2n = 54.

Setaria villosissima is a rare species that grows on granitic soils in southwestern Texas and northern Mexico. Hitchcock's (1951) report of S. villosissima growing in Arizona was based on misidentification of a specimen of S. leucopila (Reeder 1994). The villous sheaths and blades and large spikelets of S. villosissima aid in its identification.