Plants in compact, dark green tufts. Stems 1–3(–5) cm. Leaves erect-spreading, lanceolate, subulate, 2–3 mm; costa 30–55 µm wide at base, short-excurrent, rough near tip; distal laminal cells mostly subquadrate (1–2:1), incrassate; basal laminal cells elongate, alar cells differentiated, quadrate or slightly enlarged. Seta 4–6(–8) mm. Capsule exserted, slightly curved, obscurely to distinctly ribbed when dry, annulus developed, separating; peristome large, not spreading outward when dry. Spores 18–30 µm.
Phenology: Capsules mature summer.
Habitat: Siliceous rock or soil
Elevation: high-alpine elevations
Arctoa hyperborea is a rare arctic-alpine moss found on rock ledges or crevices at high elevations. It is distinguished from other species of the genus by its short-excurrent costa, shorter seta (the length is highly variable), and peristome not spreading when dry.