Plants small, slender and weak-stemmed; pale yellow to golden brown, rarely tinged with red; capitulum not especially distinct. Stems pale green to pale brown; superficial cortex of 2-3 layers of enlarged thin-walled cells. Stem leaves ovate-lingulate; 1-1.3 mm, apex broadly rounded; hyaline cells nonseptate, aporose and fibrillose in at least distal half of leaf. Branches 2-3 spreading and 2 pendent leaves not much elongated at distal end. Branch stems green; cortex enlarged, with conspicuously long-necked retort cells. Branch leaves ovate, 1-1.5 mm; straight; not or weakly undulate or recurved when dry; margins entire; hyaline cells short and wide, convex surface with 1-3 small pores per cell and on concave surface with large round wall thinnings in the cell angles; chlorophyllous cells equilateral-triangular in transverse section, broadly exposed on convex surface and just reaching to well-enclosed on concave surface. Sexual condition monoicous. Spores 27-42 µm; both surfaces smooth, proximal surface with distinct bifurcated Y-mark sculpture surrounded by distinct circular border, distal surface with distinct raised border around margins; proximal laesura usually less than 0.4 spore radius.
Capsules mature early to mid summer. Wet depressions in a variety of ombrotropic and weakly minerotrophic habitats; low to high elevations; Greenland; B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.S.; Alaska, Maine, N.J., N.Y., N.C.; Eurasia.
Sporophytes are common in Sphagnum tenellum. The delicate appearance created by the ovate and concave branch leaves as well as the large concave stem leaves make this a usually unmistakeable species.