Fimbristylis cymosa subsp. spathacea is a perennial with short rhizomes. The culms are densely tufted, stiffly erect, obtusely trigonous, and range from 5–60 cm tall and 1–2 mm in diameter. The leaves are simple and alternate, with subcoriaceous, linear blades much shorter than the culms, measuring 1–4 mm wide. The leaf blades spread to recurved, flat or with margins weakly recurved, and have an apex that is abruptly contracted and obtuse. Margins are entire, and the leaves have sheathing, persistent, thick sheaths that clothe the culm bases, which are ventrally membranous and dorsally, eventually becoming dusky brown to dark brown. The ligule is absent, and there are no stipules. The flowers form simple or compound inflorescences, up to 4 cm long and 7 cm wide, with rays 2–5 cm long. Involucral bracts are 1–3 and usually shorter than the inflorescence. Spikelets are in clusters of 2–5, rarely solitary, rusty brown or somewhat grayish brown, oblong–ellipsoid to oblong–ovoid, 3–6 mm long, and 1.5–2.5 mm wide, with an obtuse apex. The glumes are rusty brown with broadly white–hyaline, membranous margins, ovate to broadly ovate, 1–1.3 mm long, and obtuse with 3–5 nerves. The flowers are bisexual (perfect) and lack a corolla (petals). They have three stamens, and the ovary is superior with styles that are scarcely flattened, about 1 mm long, glabrous, and not fimbriate, usually with two stigmas. The fruit is a dark brown to grayish brown achene, nearly smooth, broadly obovoid, trigonous to somewhat biconvex, and 0.7–1 mm long. Each fruit contains one seed with a thin seed coat free from the pericarp, a basal embryo, and an abundant endosperm. It is found in coastal areas at elevations ranging from 0-10 meters.