Eragrostis fosbergii is a perennial grass that stands 60–100 cm tall. It has tufted, stout culms, typically originating from a geniculate woody base. The grass features simple, alternate leaves with blades 40-60 cm long and 5-10 mm wide, characterized by a scabrous upper surface and a glabrous lower surface. These coriaceous blades are flat but tend to curl inwards (involute) towards the apex. The veins run parallel, and the sheaths are glabrous, with basal ones often showing a purple coloration and being either pilose or glabrous at the throat. The ligule consists of a dense row of short hairs approximately 0.3 mm long, supplemented by deciduous hairs up to 7 mm long. The inflorescence is a pyramidal, somewhat open panicle measuring 20-40 cm long. The axils at the branches are pilose, carrying spikelets nearly to the base, with the lower branches glabrous and the upper ones scabrous. The spikelets are pale green to dark green, containing 3-5(-10) flowers, compressed, and measuring 4-5 mm long. The glumes are nearly equal in size, slender, 2-3 mm long, and deciduous, featuring a scabrous keel and long cilia, with an acuminate apex and often auriculate base. The lemmas are loosely overlapping, slightly exposing the scabrous rachilla, and are 2.5-3 mm long, occasionally ciliate, with a somewhat rough keel and acuminate apex. The palea is usually conspicuous, 2.5-2.8 mm long, with a scabrous-ciliate keel and a bifid or truncated apex. The fruit is a caryopsis. It is found in dry forests on rocky ridges between 720-920 meters.