Plants small to moderate-sized, unbranched or sparsely branched; green, golden brown to brown, capitulum small with a large and conspicuous terminal bud. Stems green to brown; superficial cortex of 2-3 layers of enlarged, thin-walled cells. Stem leaves broadly ovate, 1.2-2.2 mm; straight; apex rounded; hyaline cells non-septate, convex surface with numerous small pores (less than or equal to 1/6 cell diameter) forming a continuous row along the commissures, concave surface aporose or with a few scattered pores along the commissures and cell ends. Branches short and blunt, sometimes lacking completely and plants simplex. Branch fascicles of 1-3 branches, 1-2 of these spreading, branches usually not numerous. Branch leaves broadly ovate, 1.4-2.5(-3) mm; straight; apex rounded; hyaline cells as in stem leaf. Sexual condition dioicous. Capsule exserted, with few pseudostomata. Spores 23-35 µm; papillose on both surfaces, with indistinct Y-mark sculpture on distal surface; proximal laesura 0.5 spore radius or less.
Capsules mature late spring to early summer. Typically growing in minerotrophic habitats such as shores of lakes, ponds, streams, flarks of string mires, margins of open fens, especially seasonally flooded sites; low to high elevations; Greenland; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), Ont., Que., Yukon; Alaska, Ariz., Calif., Colo., Conn., Ind., Maine, Mass., Mich., Minn., Mont., N.H., N.J., N.Y., Ohio, Oreg., R.I., Vt., Wis., Wyo.; South America; Eurasia.
Sporophytes are uncommon in Sphagnum platyphyllum, which can be quite variable in size and the development of branch fascicles, with some forms even being simplex while other forms may have up to three branches per fascicle. The species can usually be recognized by the large stem leaves that are spreading and easily visible because of the scarcity of hanging branches. It should also be noted that in this species and S. contortum the 2-3-layered stem cortex is not an entirely consistent trait, as plants quite typical in all other respects are occasionally found with the cortex only 1-layered, at least in part.