Plants moderate-sized to fairly robust; strong-stemmed and generally compact, capitulum usually not much enlarged; greenish brown to deep golden brown; forming compact carpets in floating mats and depressions as well as dense stands on hummock sides and low hummocks. Stems brown, superficial cortical layer with spiral reinforcing fibrils visible; usually 1-2 pores per cell, comb-fibrils lacking on interior wall. Stem leaves to 1.3 × 0.7 mm; rarely hemiisophyllous; hyaline cells non-ornamented, mostly septate. Branches generally short and blunt, leaves spreading. Branch fascicles with 2 spreading and 2-3 pendent branches. Branch stems with hyaline cells non-ornamented, no or weak funnel-like projections on the interior end walls, mostly with 1 pore per cell on superficial cell wall. Branch leaves broadly ovate, 1.7 × 1 mm; hyaline cells on convex surface with round to elliptic pores along the commissures, hyaline cell walls covered with papillae where overlying chlorophyllous cells; chlorophyllous cells trapezoidal to truncate-elliptic in transverse section, equally exposed on both surfaces or less exposed on convex surface, end walls thickened. Sexual condition dioicous. Capsule with numerous pseudostomata. Spores 26-36 µm; more roughly papillose on distal surface than proximal surface, distinct raised, bifurcated-Y mark sculpture on distal surface; proximal laesura 0.5 spore radius or more.
Capsules mature mid to late summer. Very common in very poor to poor fen mire habitats where it is often a major peat former, but scarce to absent in truly ombrotrophic peatlands sites; low to moderate elevations; B.C., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que., Yukon; Alaska, Calif., Conn., Del., Ill., Ind., Maine, Md., Mich., Minn., N.H., N.J., N.Y., N.C., Ohio, Oreg., Pa., S.C., Vt., Wash., W.Va., Wis.; Eurasia.
Sphagnum papillosum is often easily field-identifiable by its rich golden brown to dark brown color and short, blunt branches. Nearly all specimens have the papillae on the branch leaf chorophyll cells but a few smooth forms have been found. Such forms will have stem leaves with divided hyaline cells whereas in the confusable species S. palustre and S. centrale such cells are rare or absent.