Plants small and slender, stiff and usually compact, capitulum small and flat-topped; typically deep reddish brown, also greenish brown in shaded habitats and in early seasonal growth, without metallic lustre when dry. Stems dark reddish brown; superficial cortical cells aporose. Stem leaves lingulate, 0.8-1.3 mm; apex broadly rounded and entire to lacerate, sometimes slightly mucronate or slightly denticulate; hyaline cells rhombic, 0-1(-2)-septate, usually efibrillose. Branches long and slender to short and compact, unranked to 5-ranked. Branch fascicles with 2 spreading and 1-2 pendent branches. Branch leaves ovate-lanceolate, 1.1-1.3 mm, straight, concave, apex strongly involute; margins entire, hyaline cells on convex surface with round to elliptic pores along the commissures, grading from small pores near the leaf apex to large pores near the base, concave surface with large round pores in proximal marginal regions of leaf. Sexual condition dioicous. Spores 17-30 µm, finely papillose on proximal surface and pusticulate on distal surface; proximal laesura less than 0.5 spore radius.
Sphagnum fuscum is common in ombrotrophic mires and alpine mountain summits where it may form small to large hummocks to 1 m in height, more infrequently in weakly minerotrophic mires and richer fens.
Sporophytes are common in Sphagnum fuscum, which is associated with S. angustifolium, S. fallax, S. magellanicum, S. papillosum, and more infrequently with S. teres, and S. warnstorfii in richer sites. Very widespread but generally easily recognized, it is the only small brown hummock-forming species of sect. Acutifolia over most of its range. There are some significant variations in this species. The stem leaves can vary from having a rounded, entire apex to having a somewhat flat and lacerate apex. The branches also vary from being unranked and slender to 5-ranked and blunt. The color also can vary from a light to a dark brown. There does not seem, however, to be any consistent pattern to these variations and thus no taxonomic recognition has been given to them. See also discussion under 73. S. flavicomans.