Polytrichastrum alpinum grows to form a loose, muddy green or bluish-green to brownish lawn.

The leaf rib is cut in the upper part, emerging from the back as a short spike from the blade tip.

The moss grows in a variety of habitats, from shaded to sunny sites, and on fresh to moist, nutrient-poor, calcareous and base-poor soils.

In Europe, it is often found in arctic and boreal regions, while further south it is restricted to montane landscapes and alpine areas.

Key distinguishing features are that the spore capsules of the Alpine moss are at the bottom of large single-celled stomata; the capsule outer wall has smooth cells; the leaf sheath is much longer than that of Pogonatum urnigerum and the leaf blade is narrower and more spiky.