Stems long-creeping, ca. 1 cm diam. Leaves monomorphic, evergreen, 4--8 cm apart, to ca. 3 m. Petiole straw-colored, to 1.3 m × 0.8--1.2 cm, at base sparsely set with brown, lanceolate, hairy scales. Blade to ca. 1.7 m, broadest at or near base, gradually to somewhat abruptly tapered to pinnatifid apex. Pinnae (15--)20--45 × 2--3.5(--4.8) cm, incised mostly 3/4--9/10 of width; segments oblique, curved, 2--4 basal basiscopic ones (those facing base of petiole) on proximal pinnae greatly reduced or wanting; proximal 1--2(--3) pairs of veins from adjacent segments connivent at sinus. Indument abaxially of hairs 0.1 mm on costae and veins, also of brownish, hairy scales on costae; blade tissue glabrous on both sides. Sori round, medial to supramedial; indusia pinkish to reddish brown, glabrous; sporangia glabrous. 2 n = 72.

Mixed swamps and old logging roads; 0--50 m; Fla.; West Indies in the Greater Antilles, St. Christopher (St. Kitts).

In the flora Thelypteris grandis var. grandis is known only from one population in Collier County, Florida.