Stems short-creeping; scales bicolored, centers dark reddish brown, margins pale tan, erose-ciliate. Leaves arching or sometimes pendent, closely spaced, 20--110 cm. Petiole 1--3 mm diam., glabrous, occasionally glaucous. Blade trowel-shaped, pinnate, 12--60 × 12--60 cm, gradually reduced distally, glabrous; proximal pinnae 3-pinnate; rachis straight, glabrous, not glaucous. Segment stalks 1--5 mm, with dark color ending abruptly at segment base, terminating in cupulelike swelling at base of segment (unlike any other species of Adiantum in the flora). Ultimate segments fan-shaped or rhombic, about as long as broad; base cuneate; apex rounded or acute, lobed, lobes separated by narrow incisions 0.5 mm wide. Indusia transversely oblong to crescent-shaped, 0.5--2 mm, glabrous. Spores mostly 40--58 µm diam. 2 n = 60.

Sporulating throughout the year. Restricted to moist, shaded, limestone ledges, sink walls, and grottoes in the flora; 0--50 m; Fla.; e,s Mexico; Central America in Guatemala, Honduras, Nicaragua, Costa Rica; South America in Venezuela.

Adiantum tenerum is readily distinguished from other species in the flora by the ultimate segments conspicuously articulate to the stalks.