Eragrostis grandis is a perennial grass forming large tufts reaching up to 2 meters in height. The culms are erect, measuring up to 1 centimeter in diameter at the base, and are glabrous. This species features simple, alternate leaves with flat blades that typically curl inward toward the slender apex, usually 5-10 millimeters wide. The sheaths are glabrous, with the throat either pilose or glabrous, and the ligule consists of a dense row of hairs 0.2-0.4 millimeters long. The inflorescence is an open, paniculate structure that is oblong and can extend up to 50 centimeters in length. Its branches are widely spaced and fasciculate, ascending and usually bearing spikelets nearly to the base. The axils are pilose, with the axis and branches being smooth in the lower part and scabrous in the upper part. The spikelets are 5-10-flowered, compressed, and 5-9 millimeters long, with rachilla internodes approximately 0.8 millimeters long and scabrous and lateral pedicels about 1 millimeter long. The glumes are scabrous, 2.5-3 millimeters long, with an acuminate apex. Lemmas are loosely imbricate, revealing the rachilla, about 3 millimeters long, glabrous, with a scaberulous keel and an acuminate or acute apex; the palea is slightly shorter than the lemma, with a scabrous-ciliate keel. The fruit is a reddish-brown caryopsis, ovoid to oblong, measuring 0.5-1.5 millimeters long. It is found on moist to wet slopes in ravines and mesic forests, typically in partial shade between 170-2,044 meters.