Cyperus pennatiformis var. bryanii is a perennial sedge with short, ligneous rhizomes covered in brown scales and fibrous remnants. The culms are 40–120 cm tall and 3–7 mm in diameter at the lower part, acutely trigonous with slightly concave sides and smooth. It bears 3–5 leaves per culm, with subcoriaceous, flat, linear blades as long as or somewhat longer than the culm, measuring 8–17 mm wide, and featuring a finely septate-nodulose lower surface. The leaves are entire at the margins and sheathed with brown to reddish brown sheaths that eventually split into fine parallel fibers. The flowers are a moderately dense, umbelliform inflorescence measuring 4–15 cm long and 5–25 cm wide, with 5–18 spreading rays 2–15 cm long, which may be unbranched or once branched. The spikes are cylindrical to oblong-ellipsoid, 2–6 cm long and 1.5–2.7 cm wide, densely populated with numerous spikelets. It has 5–9 involucral bracts longer than the inflorescence, with the lower ones reaching up to 35 cm long and 22 mm wide. The spikelets are yellowish-brown or grayish-brown, 8–12-flowered, spreading to reflexed, flattened, slightly turgid, narrowly oblong to oblong-lanceolate, 8–12 mm long, 3–4 mm wide, with a subacute apex. The rachilla is narrowly winged and jointed at the base. The glumes are chartaceous with membranous margins, closely imbricate, grayish-brown, slightly spreading, narrowly oblong, 4–5.5 mm long, 1.5–2.2 mm wide, finely 4-nerved on each side of the midnerve, and the midnerve is prolonged into a short mucro. The flowers are bisexual (perfect) and lack a corolla. The fruit is an ellipsoid, trigonous achene 1 mm long, containing a single seed. The seed coat is thin and free from the pericarp, with a basal embryo and abundant endosperm. It is found in open sites on coastal dunes at elevations between 0-15 meters.