Panicum tenuifolium is a cespitose (growing in clumps) perennial grass ranging in height from 25 to 120 cm. It features unbranched culms that are papillose-pilose, with nodes varying from glabrous to pilose and fibrous roots. The leaves are primarily arranged along the stem, with blades measuring 5-20 cm long and 0.1-0.5(-0.7) cm in width. These blades are usually loosely involute but can also be mixed with flat ones or entirely flat, exhibiting a papillose-pilose texture, particularly on the lower surface. The sheaths share this texture and are complemented by a ciliate ligule, a rudimentary membrane fringed with hairs measuring 0.2-1.1 mm in length. It produces terminal panicles that are 5-30 cm long, with branches that extend from ascending to widely spreading and are typically solitary. These branches are scabrid to pilose, with spikelets (pictured) that are 2.1-3.6 mm long, glabrous, and either acute or acuminate. The glumes are as long as the spikelets, with the first floret being sterile and its lemma nearly matching the glumes in length but bearing 5-7 nerves. The second floret is glossy, smooth, and slightly shorter than the first lemma, featuring elliptic to elliptic-lanceolate shapes. It thrives in various environments, including talus slopes, lava flows, mountain parklands, or openings in dry forests between 1,200 to 2,300 meters.