Plants 1-2(-7) cm. Stems reddish brown or red. Leaves green to yellow-green, crisped and somewhat coiled around stem, slightly undulate or contorted when dry, elliptic-lanceolate, elliptic, ovate, or ovate-lanceolate, 1.5-3(-4.5) mm; base long-decurrent; margins green, occasionally brownish or reddish brown, 2-stratose, toothed to below mid leaf, teeth variable, paired or sometimes single, small, sharp or blunt; apex acute, rounded-acute, or obtuse, apiculate or cuspidate, cusp weakly toothed or smooth, occasionally ending in long terminal cell; costa percurrent, sometimes excurrent or subpercurrent, distal abaxial surface lightly toothed or sometimes smooth; medial laminal cell shape variable, ± isodiametric or slightly elongate, (10-)14-22(-25) µm, slightly smaller towards margins, in longitudinal, rarely diagonal rows, not or weakly collenchymatous, blue postmortal color absent; marginal cells linear, in (1-) 2-3(-4) rows. Sexual condition dioicous. Seta single. Capsule yellow-brown or brown, 2-5 mm; operculum rostrate; exostome greenish yellow. Spores 20-40 µm.

Capsules mature summer. Calciphile, moist rock, soil, bark, along streams; low to high elevations; Greenland; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., Que., Yukon; Alaska, Ariz., Ark., Calif., Colo., Conn., Fla., Ill., Ky., Maine, Mass., Mich., Minn., Mo., Mont., Nebr., N.Mex., N.Y., N.C., N.Dak., Ohio, Oreg., Pa., S.Dak., Vt., Va., Wash., Wyo.; Europe; Asia.

Mnium thomsonii is morphologically variable, especially in leaf shape, degree and type of marginal toothing, and costal length and toothing. Small plants often lack costal teeth and margins can be nearly smooth. However, the small laminal cells, the smallest in the genus, are diagnostic. The name M. orthorrhychum Müller Hal., often used for this taxon, is an illegitimate homonym (T. J. Koponen 1972).