Plants slender to moderately robust, capitulum flat-topped to somewhate convex, terminal bud rather conspicuous, slightly to distinctly stellate; green to variegated green and brown-red to red-brown, lacking metallic sheen when dry. Stems pale or pale with red-brown portions particularly during late autumn; superficial cortical cells mostly with one ± circular to transversely elliptical pore close to distal cell wall, occasionally superficial cells with 2(-3) pores with most occupying more than half the cell width. Stem leaves broadly lingulate-spathulate to spatulate, widest at base and/or distal third, usually narrower in the mid region, 1-1.3 mm, apex narrowly to broadly truncate and ± fimbriate-lacerate, border narrow distally and expanded proximally to 0.25 width or more; hyaline cells efibrillose, irregularly rhombic to subquadrate, many cells 2-4-septate. Branches unranked. Branch fascicles with 3(4-5) spreading branches and 1-2 pendent branches. Branch leaves ovate-lanceolate, 1.1-1.4 mm, slightly concave, straight, apex involute; hyaline cells on convex surface grading from aporose to few small round pores near apex to numerous semi-elliptic pores along commissures in middle and base of leaf, concave surface with a few large irregular to circular pores (1-4) occupying most of the cell width. Sexual condition autoicous. Spores 20-27 µm, moderately granulate-papillose on both surfaces, distinct raise Y-mark sculpture on distal surface; proximal laesura less than or equal to 0.5 spore radius.

Capsules mature mid to late summer. Shaded areas in humid, spruce forests; low to moderate elevations; B.C., Nfld. and Labr. (Nfld.); Alaska, Wash.; Europe.

The sporophytes of Sphagnum rubiginosum are common. The geographic distribution is unclear because of the species’ recent description and possible confusion with S. girgensohnii. It is clearly distinct from S. girgensohnii in coloration, number of spreading branch fascicles, the common occurrence of sporophytes (rarely found with S. girgensohnii, and spore morophology. The conspicuous three spreading branches will separate this from all species of sect. Acutifolia except S. quinquefarium. The latter has 5-ranked branch leaves, while those of S. rubiginosum are unranked.