Rhizomes horizontal, brownish, thick, praemorse, fleshy. Scapes 1–3, round in cross section, 0.8–2.5 dm, slender to stout, glabrous. Bracts held well above ground, sessile; blade green to bluish green, strongly to sparsely mottled, mottling becoming obscure with age, oval to suborbicular, 4–10 × 2–8 cm, base broadly attached, apex rounded-acuminate to bluntly parallel sided-acuminate (rounded basally to its broad attachment). Flower erect, odor pungent, spicy; sepals displayed above bracts, spreading, green, variously streaked with maroon, lanceolate-oblanceolate, 9–35 × 4–8 mm, margins entire, apex rounded-acuminate; petals long-lasting, erect, ± connivent, ± concealing stamens and ovary, maroon, brownish maroon, green, or yellowish green, not spirally twisted, oblanceolate to elliptic, occasionally almost orbicular, 1.7–3.5 × 0.7–2 cm, thick-textured, narrowed near basal attachment (but not truly clawed), margins entire, apex gradually rounded-tapered to acute; stamens straight, 10–23 mm; filaments red-purple, 2–5 mm, dilated basally; anthers erect, straight, gray-purple, 9–16 mm, thick, dehiscence introrse; connectives purplish brown, straight, projecting 2–5+ mm beyond anther sacs; ovary greenish white basally, purple distally, ovoid to globose, 6-angled, pyramidally narrowed to stigmas, 4–8.5 mm; stigmas erect, divergent-recurved, distinct, purple, subulate, 1–5 mm, ± fleshy. Fruits baccate, dark greenish purple, odorless, subglobose, 6-angled, angles somewhat winglike, pulpy, not juicy. 2n = 10.
Flowering spring (Mar–early May). Rich woodlands, limestone districts, calcareous soils, floodplains, riverbanks, clayey alluvium, less fertile soils, high, dry limestone woods, persists under light pasturing, in fencerows and brushy areas after lumbering; 100–300 m; Ala., Ark., Ill., Ind., Kans., Ky., Md., Mich., Mo., N.Y., N.C., Ohio, Okla., Pa., Tenn., Va., W.Va.
Trillium sessile is rather uniform throughout its range, with few color forms