Carex alligata is a moderately densely tufted perennial plant with short, woody rhizomes. The culms, or stems, are trigonous and can grow between 60 and 150 cm tall, featuring a scabrous texture on the angles just below the inflorescence. Its leaves are simple, arranged in three ranks, with linear blades nearly as long as the culm, measuring 8 to 15 mm in width, and slightly plicate. The apex of the leaf blade tapers gradually, becoming long-acuminate and scabrous at the end, while the margins remain entire. The leaves are sheathed, with the sheaths varying from pale greenish to pale brown and often longer than 20 cm; these sheaths are hyaline on the ventral side and split into parallel fibers. Accompanying cataphylls are dark brown to purplish brown, which disintegrate into brown fibers over time. The flowers are organized into spikes, with 5 to 9 closely spaced and almost finger-like arrangements. The terminal spike is staminate and ranges in color from brown to rusty brown, cylindrical in shape, and between 5 to 17 cm in length and about 3 mm wide. The lateral spikes are androgynous, with a short staminate distal part, and occasionally, the lower lateral spikes are pistillate. These cylindrical spikes range from 5 to 12 cm in length and 4 to 5 mm in width, with a short lax peduncle, making the upper spikes nearly sessile. The plant’s bracts are non-sheathing, with the lower ones significantly surpassing the inflorescence; the lowest bract can reach up to 60 cm long. The pistillate glumes are reddish to purplish brown, membranous, and elliptic to oblong–obovate, measuring 3–4 mm long and 1–2 mm wide. These acute glumes have a green midnerve extending into a scabrous awn measuring 0.5–0.8 mm. The perigynia of the plant are dull, yellowish-green, sometimes with a reddish-purple or brown tinge, and thick, ranging from membranous to chartaceous. They are broadly obovoid to broadly ellipsoid, spreading at 2–2.5 mm in length and 1–2 mm wide, glabrous, usually having one or two nerves on the dorsal side, with two lateral ribs. They are abruptly contracted to the short-beaked apex with a shallowly bidentate orifice. The flowers are unisexual, with the corolla absent. The ovary is superior with two stigmas. The fruit is an achene, obovate, and lenticular in shape. It contains a single seed with a thin coat free from the pericarp. The embryo is located at the base and is surrounded by an abundant endosperm. It is found in wet forests, alongside stream banks, and in other moist sites between 305-2,225 meters.