Plants small, gracile, and rather delicate in appearance, green to brownish green. Stems to 2 cm. Leaves 2.5-6 × 0.7-1.5 mm, lanceolate to ovate-lanceolate, plane or rarely weakly undulate, teeth few and scattered on abaxial surface of lamina or absent, the apex acute, proximal leaves somewhat obtuse; border narrow, with small, rather distant marginal teeth; costa subpercurrent to percurrent, with several abaxial teeth near apex; lamellae 2-5, 4-7 cells high; median leaf cells 19-24 µm wide, laxly subquadrate to irregularly hexagonal, evenly thin-walled, not or weakly collenchymatous, becoming somewhat larger adjacent to the costa, smooth. Sexual condition dioicous; male plants about same size as female, perigonial bracts broad, forming a cup-like inflorescence, sometimes more than one per plant. Seta 1(-2) per perichaetium, to 2 cm. Capsule 1-2 × 0.5-0.8 mm, erect to inclined, straight to somewhat curved; operculum 1.5-2 mm. Spores 15-30 µm.
Capsules mature late summer (Aug-Sep). Clay or sandy soil, especially in exposed habitats, beside streams, in roadside ditches, along trails and clearings in woodlands; low to moderate elevations; B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., Ont., P.E.I., Que.; Ill., Maine, Minn., Wis.; n, c Europe; n Asia.
Atrichum tenellum is a species with few distinguishing features, evidently rather rare (or seldom collected), with comparatively few lamellae, a narrow leaf border, thin-walled median leaf cells without trigones, and abaxial teeth few or absent. Previously confused with A. crispum, it is easily distinguished by the absence of papillae on the marginal teeth and border. In addition, the lamellae of A. crispum are always low and discontinuous.