Plants small to medium, in loose tufts. Stems with cortical cells thin-walled. Leaves loosely to densely imbricate, erect-incurved at the tips, often ± secund, obtusely cucullate; sheath broadly elliptic, contracted to the blade; marginal lamina 2--6 cells wide, slightly broader and inflexed in the distal part of blade; perichaetial leaves similar to the foliage leaves. Seta straight. Capsule (4--)5--6-angled to terete; peristome teeth 50--64, slender, of uniform size.

Damp gravelly soil and rocks, snowbed communities and beside snow-melt streams; middle to upper elevations; Greenland; Alta., B.C., Nunavut (Baffin Is.), Nfld. and Labr., N.W.T., Que., Yukon; Alaska (including the Aleutians), Mont., Utah, Wash., Wyo.; Europe; Asia (Russia in s Siberia, Japan); Atlantic Islands (Iceland).

Even when sterile, P. sexangulare can usually be recognized by secund, tubulose leaves with an obtusely cucullate apex. Polytrichastrum alpinum var. septentrionale is similar, but has papillose lamellae, and the leaf apex is not cucullate. There has been a long-standing confusion of the names Polytrichum septentrionale, P. norwegicum, and P. sexangulare dating from the earliest days of bryology, and still met with in older collections in herbaria. Hedwig's P. norwegicum was briefly in vogue as a name for this species, but the type of P. norwegicum is a form of P. alpinum (Gary L. Smith 1971).