Schoenoplectus tabernaemontani is a perennial sedge with coarse, horizontally creeping rhizomes measuring 0.8–1.5 cm in diameter. The stems (culms) are solitary or occur in small groups, arising in a row along the rhizome. They are dull green, terete (cylindrical), and lack nodes. The culms range from 70 to 300 cm in height and are 5–10 mm in diameter at the base. The leaves are simple and alternate, typically reduced to 3–5 bladeless sheaths, though some may have blades up to 10 cm long. The margins are entire. The sheaths range from 10 to 40 cm in length, with the lower ones being dusky brown and disintegrating into reticulate fibers, while the upper ones are pale green with an oblique orifice. Stipules are absent. The flowers are arranged in anthelate, compound to partially decompound inflorescences that are recurved to nodding, bearing numerous spikelets. The primary rays measure 3–8 cm long, while the secondary rays can reach up to 2 cm in length. The involucral bracts number 2–3, with the lowest being subulate, erect, and 1–3 cm long, and the others being membranous and scale-like. The spikelets occur solitarily or in clusters of 2–3, are rusty brown, and range from ovoid to ovoid–ellipsoid in shape, measuring 6–15 mm long and 3–4 mm wide, with an acute tip. The glumes are ovate to elliptic, 2.5–3.2 mm long, membranous, and have reddish-brown resinous spots on the upper median portion. Their margins are broadly white–hyaline and ciliate toward the apex, with a green midnerve that extends as a mucro at the obtuse to shallowly emarginate apex. The bristles, numbering 2–5, are shorter than or as long as the achene and retrorsely spinulose–scabrous. The flowers are bisexual (perfect) and lack a corolla (petals). There are three stamens. The ovary is superior, with two stigmas. The fruit are grayish-brown, broadly ovoid achenes measuring 1.8–2.5 mm long and are plano-convex. Each achene contains a single seed with a thin seed coat free from the pericarp, a basal embryo, and an abundant endosperm. It is found in fresh and saltwater marshes from sea level to 1,220 meters.