Plants fairly slender to moderate-sized, pale green to yellowish, or reddish brown in sun-grown forms; forms loose to dense carpets. Stems pale green to red-brown; 3-4 layers of superficial cells. Stem leaves generally larger than branch leaves, 1.3- 1.8 × 0.8-1 mm; elliptic to lingulate-spatulate, widest above middle, hyaline cells nonseptate. Branches long-cylindrical, branch leaves terete to sometimes distinctly squarrose in shade forms. Branch fascicles typically with 3 spreading (sometimes 2) and 2 pendent branches. Branch stems with single layer of cortical cells. Branch leaves 1-1.4 mm, ovate to ovate-lanceolate, gradually narrowed to an involute tip, hyaline cells somewhat bulging on concave surface and nearly plane on convex surface, with 4-8 large, elliptic, unringed pores per cell on convex surface and 1-4 irregularly rounded pores per cell on concave surface, internal commissural walls smooth to rather strongly papillose, chlorophyllous cells ovate-triangular with the widest part at or close to the convex surface. Sexual condition dioicous. Spores 21-26 µm; proximal and distal surfaces smooth, papillae indistinct; proximal laesura 0.5-0.6 spore radius.
Sporophytes uncommon, capsules mature late spring to early summer. Strongly minerotrophic, in open to medium rich fens, less frequent in coniferous mires, characteristic species of rich, weakly acidic to slightly basic mires; low to high elevations; Greenland; Alta., B.C., Man., Nfld. and Labr. (Nfld.), N.W.T., N.S., Nunavut, Ont., Que., Sask., Yukon; Alaska, Calif., Colo., Conn., Idaho, Ill., Ind., Iowa, Maine, Mass., Mich., Minn., Mont., N.H., N.J., N.Y., N.Dak., Ohio, Oreg., Pa., Vt., Wash., Wis., Wyo.; Eurasia.
Shade forms of Sphagnum teres are often squarrose but these are usually considerably smaller than S. squarrosum. For other distinctions between these species, see discussion under the latter.