Calamagrostis hillebrandii is a perennial grass with slender, short rhizomes. It features tufted, erect, glabrous culms that stand 30–50 cm tall. The leaves are alternate, firm, and flat to involute, with midculm blades measuring 10–15 cm long and 2–4 mm wide (when flat), tapering to an acute, hard apex. The upper surfaces of the leaves are scabrous, while the lower surfaces are glabrous or nearly so. Sheaths are glabrous, enveloping the internodes longer than themselves, with tightly overlapping margins and puberulent basal sheaths. A membranous, erose ligule around 1 mm long is present at the leaf junction. The flowers are in an ovate, somewhat loose, paniculate inflorescence ranging from 5 to 30 cm in length and 3 to 7 cm in width, ascending to spreading branches that are usually pubescent near the base. The rachilla, 1.5-2 mm long, is silky villous, extending to the lemma tip and sometimes forming an awn up to 4 mm long. Glumes are nearly equal in length, enclosing the floret and opening to reveal a 4-5 mm long lemma, which is firm, faintly 5-nerved, and short-pilose at the base, ending in a hyaline, 4-toothed apex with slender awns up to 1 mm long. The subapical awn is geniculate and scabrous, measuring 8-9(-10) mm long. The palea is about 3.5-4 mm long, sharply 2-toothed, with scaberulous keels and a laterally pubescent callus. The fruit is a caryopsis. It is found in open bogs at elevations ranging from 1,220-2,990 meters.