Plants perennial, creeping, rhi-zomatous. Stems decumbent and ascending, branched, smoothly 4-angled, 10-40 cm, glabrous. Leaves sessile; blade narrowly elliptic, elliptic-lanceolate, or oblanceolate, 0.5-2(-3) cm × 2-10(-13) mm, base cuneate, margins thin with reticulate venation, entire, apex acute, glabrous, slightly ciliate basally. Inflorescences axillary with 1-5-flowered cymes in mid and distal axils of foliage leaves; bracts lanceolate, ca. 1 mm, scarious with green midrib. Pedicels 5-30 mm, glabrous. Flowers ca. 6 mm diam.; sepals 5, 3-veined, lanceolate-triangular, 2.5-3.5 mm, margins scarious, apex acute, glabrous; petals 5, 1.5-3 mm, shorter than (rarely equaling) sepals, blade apex 2-fid almost to base, with widely divergent lobes; stamens 10; styles 3, ca. 1 mm. Capsules green, ovoid, 2.5-3.5 mm, equaling sepals, apex broadly acute; carpophore absent. Seeds pale reddish brown, ± reniform, 0.3-0.4 mm diam., with small tubercles. 2n = 24. Flowering spring-early summer. Streamsides, flushes, wet tracks, ditches; 0-300 m; St. Pierre and Miquelon; B.C., N.B., Nfld. and Labr. (Nfld.), N.S., P.E.I., Que.; Del., D.C., Ga., La., Maine, Md., Mass., Minn., N.H., N.J., N.Y., N.C., Ohio, Pa., Tenn., Vt., Wash., W.Va.; Europe; introduced in South America (Chile). Stellaria alsine is presumed to be native in eastern North America but has been introduced elsewhere in North America and Chile.