Plants perennial, matted. Tap-roots filiform; rhizomes weak, slender, 0.5-15+ cm. Stems 5-40+, flowering stems erect to ascending, green, 2-8(-10) cm; internodes terete to angular, 2-4 times as long as leaves, shiny, glabrous or occasionally sparsely stipitate-glandular at distalmost internodes. Leaves connate basally, with scarious sheath 0.2-0.3 mm, sessile; blade weakly to prominently 1-veined, narrowly oblong or elliptic to ovate-lanceolate, 2-12 × 1-3 mm, subsucculent, margins thickened, herbaceous, shiny, smooth, apex blunt to acute, prominently pustulate or not, glabrous; axillary leaf clusters absent. Inflorescences terminal, open, leafy, 1-3-flowered cymes or often flowers solitary. Pedicels erect in fruit, 1-10 mm, minutely retrorsely pubescent to stipitate-glandular. Flowers: sepals green, 3-veined, 2 lateral veins 2- 4 times as long as midvein, more prominent in fruit, not keeled, ovate (herbaceous portion ovate), 2.5-4 mm, not enlarging in fruit, apex obtuse or somewhat acute, not pustulate, glabrous; petals oblanceolate, 3-4 mm, ± 1 2 times as long as sepals, apex acute. Capsules tightly enclosed by calyx, broadly ellipsoid, 4-4.5 mm, ca. 11/ 4 times as long as sepals. Seeds 12-18, brown, reniform, compressed, 0.5-0.7 mm, shiny, faintly rugulose. 2n = 40, 44 (both Europe). Flowering spring-summer. Moist, calcareous gravels and rock crevices; 0-100 m; Greenland; St. Pierre and Miquelon; Man., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., P.E.I., Que.; Europe. Arenaria humifusa is similar to A. longipedunculata, but is smaller in some dimensions than that species.