Plants moderate-sized to robust, somewhat lax in shade forms to quite compact and stiff in open grown forms; green to pinkish green to reddish purple; forms lawns in shaded habitats and low to moderately tall, dense hummocks in open habitats. Stems green to purplish red, superficial cortical cells with spiral reinforcing fibrils clearly visible, usually 1 or 2 pores per cell, comb-fibrils lacking on interior wall. Stem leaves to 2 × 0.7 mm; rarely hemiisophyllous; hyaline cells non-ornamented, mostly nonseptate. Branches long and tapering to short and pointed, leaves loosely imbricate. Branch fascicles with 2-3 spreading and 2-3 pendent branches. Branch stems with hyaline cells non-ornamented; no or weak funnel-like projections on the interior end walls, large round pores on superficial cell walls. Branch leaves broadly ovate, to 2 × 1 mm or more wide, broadly ovate, hyaline cells non-ornamented, convex surface with round to elliptic pores along the commissures; chlorophyllous cells short-elliptic in transverse section and well-enclosed on both surfaces. Sexual condition dioicous. Capsule with numerous pseudostomata. Spores 22-30 µm; roughly papillose to nearly smooth, with distinct Y-mark sculpture on distal surface; proximal laesura 0.5-0.8 spore radius.
Capsules mature mid summer. Ecological amplitude very wide, ombrotrophic to rich fen peatlands, forested and open mires; low to high elevations; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Ala., Alaska, Ark., Calif., Conn., Del., Fla., Ga., Idaho, Ill., Ind., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., N.H., N.J., N.Y., N.C., Ohio, Oreg., Pa., S.C., Tenn., Tex., Vt., Va., Wash., W.Va., Wis.; South America; Eurasia.
As the only boreal species of the section with a reddish purple color, Sphagnum magellanicum is usually easy to identify. The branch leaf chlorophyll cells are capable of being confused only with those of S. alaskense, which are less enclosed on both surfaces, and S. centrale, which has thickened end walls on the chlorophyll cells that give them a narrow exposure on the concave surface.