Plants perennial (rarely annual), tufted to mat-forming, often rhizomatous. Stems: flowering stems erect from decumbent base, branched proximally, 10-45 cm, softly pubescent, eglandular with straight hairs; nonflowering shoots, when present, produced proximally, decumbent, rooting at nodes, branched, 5-20 cm, often subglabrous with alternating lines of eglandular hairs; small axillary tufts of leaves usually absent. Leaves not marcescent; blade 10-25(-40) × 3-8(-12) mm, densely covered with patent to ascending, colorless, long, eglandular hairs; leaves of flowering shoots in distant pairs, sessile, blade elliptic to ovate-oblong, apex subacute; leaves of sterile shoots pseudopetiolate, often spatulate, blade oblanceolate, apex obtuse. Inflorescences lax, 3-50-flowered cymes; bracts lanceolate, reduced, herbaceous, eglandular-pubescent, distal often with narrow, scarious margins. Pedicels somewhat curved distally, 2-10(-20) mm, longer than sepals, densely pubescent with patent, eglandular, rarely glandular hairs. Flowers: sepals ovate-lanceolate, 5-7 mm, margins narrow, apex acute, scarious, pubescent with eglandular, rarely glandular, hairs; petals oblanceolate, 1-1.5 times as long as sepals, apex deeply 2-fid; stamens 10, occasionally 5; styles 5. Capsules narrowly cylindric, curved, 9-17 mm, ca. 2 times sepals; teeth 10, erect, margins convolute. Seeds reddish brown, 0.4-1.2 mm, bluntly tuberculate; testa not inflated, tightly enclosing seed. 2n = 122-152, usually 144.

Cerastium fontanum is a highly variable and complex species. It often has been reported as C. vulgatum Linnaeus, an ambiguous name; see discussion under the genus.