Bolboschoenus maritimus subsp. paludosus is a robust perennial with horizontal, long-creeping rhizomes bearing tubers. The stems are repent rhizomes clothed with brown scales, forming ligneous tubers at culm bases and nodes. The culms are erect, with several leafy, trigonous, smooth nodes ranging from 30–150 cm tall. The leaves are simple and alternate, with linear blades that are 3–8 mm wide. The leaf apex is acuminate, and the surfaces are scabrous on the distal margins and midrib toward the apex on the lower surface but otherwise smooth. The margins are entire. Leaves are sheathing at the base, with brown basal sheaths, and stipules are absent. The flowers occur in moderately dense, anthelate inflorescences consisting of terminal, congested, head-like clusters of 3 to numerous spikelets, with 1–4 rays, each terminated by a head-like cluster of spikelets. The rays can be up to 5 cm long. Involucral bracts number 2–5, with the lower ones much surpassing the inflorescence, the longest reaching up to 30 cm. Spikelets are pale brown, ellipsoid, ovoid–ellipsoid, or subcylindrical and terete, measuring 1–2.5 cm long and 0.6–1 cm in diameter. The glumes are pale brown with pale margins, thin, membranous, narrowly ovate, 5–6.5 mm long, and about 3 mm wide, tapering to an acute, bifid apex, pubescent externally, with 1(3) nerves, the midnerve prolonged ca. 2 mm beyond the apex. Flowers are bisexual (perfect), with 2–6 perianth bristles up to half as long as the achene, which is caducous. The corolla is absent, and there are three stamens. The ovary is superior. The fruit is an achene that is brown, glossy, and obovate, measuring 3–4 mm long and 1.8–2.2 mm wide, lenticular, puncticulate, with an apiculate apex. Each fruit contains one seed, with a thin seed coat free from the pericarp, a basal embryo, and an abundant endosperm. It occurs in coastal sites from sea level to approximately 6 meters.