Rhizomes long creeping, blackish brown, glabrate, with sparse brownish, broadly ovate, membranous scales, more densely scaly at apex. Fronds distant; fertile fronds 10-30 cm; stipe brown or chestnut-colored, upward stramineous, 6-17 cm, base with sparse brownish, ovate scales, upper part with only few scales; lamina 3-pinnate with secondary pinnules pinnatipartite, ovate, 5-13 × 4-7 cm, base broadly cuneate, apex acuminate; pinnae 4 or 5 pairs, alternate, ascending, stalk 3-5 mm; basal pinnae largest, narrowly deltoid-ovate, ca. 7 × 2.5 cm, base broadly cuneate, apex acuminate; pinnules 5-9 pairs, alternate, much ascending, ca. 8 mm apart, oblong-ovate, base subtruncate, equilateral, shortly stalked, apex acuminate; basal basiscopic pinnules of lowest pinnae longest, up to 3.2 cm; secondary pinnules 4 or 5 pairs, oblong, 3-6 mm, gradually narrowed to apex, base subsessile, not decurrent, apex obtuse; pinnule segments small, 2 or 3 pairs, approximate, broadly ovate, decurrent to narrow costular wing, subentire, revolute at margin, apex rounded-obtuse; second basal pinnae ca. 3 cm apart from basal pinnae. Veins pinnate in segment, free; rachis, costae, and midribs glabrate. Sori orbicular, abaxial on veins; indusia brown, persistent. Perispore foveolate or reticulate with rugate folds.
Cystopteris modesta is very similar to C. montana but distinct from it in the revolute segments and foveolate or finely reticulate perispore.