Panicum lineale is a densely cespitose perennial grass growing between 55 and 126 cm tall. The stems, or culms, are erect, unbranched, and glabrous. The leaves are simple, alternate, primarily basal, and often strongly distichous. Leaf blades range from 59 to 85 cm long and 0.4 to 0.9 cm wide when unrolled, but they are usually involute. The apex is long-acuminate. The upper surfaces are densely velvety pubescent at the base behind the ligule but otherwise glabrous, while the lower surfaces are entirely glabrous. Veins are parallel. Sheaths are either glabrous or puberulent toward the apex. The ligule is a rudimentary membrane with cilia measuring 0.4 to 0.5 mm long. Stipules are absent. The flowers are arranged in panicles measuring 20 to 35 cm long and 6 to 10 cm wide, with a glabrous rachis. The branches spread at acute angles, and the pulvini are pubescent. The spikelets (pictured) measure 4.1 to 4.8 mm long and are linear-lanceolate, acuminate, and glabrous. The rachillar internodes are prominent. The glumes are unequal: the first glume measures 1.7 to 2.6 mm long, is 3- to 7-nerved, and has an acute apex, while the second glume is 4 to 4.5 mm long and 9-nerved. The first floret is sterile, with a lemma as long as the spikelet, similar to the second glume, and 7- to 9-nerved. The first palea is 1/5 to 1/2 as long as the first lemma. The second floret is fertile, 2.5 to 2.9 mm long and 0.7 to 0.8 mm wide, linear-elliptic, cartilaginous, smooth, glossy, and acute. The anthers are three in number, orange, and measure 2.2 to 2.4 mm long. The fruit is a caryopsis. It occurs on steep, moist slopes in open habitats between 180-900 meters.