Plants perennial, not matted. Taproots filiform; rhizomes slender, 15+ cm. Stems 5-15, weakly erect, green, 25-90 cm; internodes angular to grooved, 2/ 3-3 times as long as leaves, shiny, glabrous except for fine hairs at nodes. Leaves connate basally, with scarious sheath 0.2-0.5 mm, sessile; blade 1-veined (vein prominent abaxially), linear to linear-lanceolate, 20-55 × 2-7 mm, herbaceous to subsucculent, margins ± flat, scarious, shiny, ciliate, apex acute or acuminate to cuspidate, sometimes pustulate, sparsely pubescent on adaxial surface; axillary leaf clusters absent. Inflorescences axillary, solitary flowers. Pedicels reflexed in fruit, 20-50 mm, glabrous. Flowers: sepals green, obscurely 1-veined, not keeled, broadly elliptic (herbaceous portion narrowly elliptic), 2.8-3.5 mm, to ca. 4 mm in fruit, apex obtuse to rounded, not pustulate, minutely ciliate basally; petals ovate, 5-6 mm, 1 1/2-2 times as long as sepals, apex rounded. Capsules tightly enclosed by calyx, ovoid, 4 mm, ca. equaling sepals. Seeds 15-20, dark brown, reniform, compressed, 0.8-0.9 mm, shiny, smooth. Flowering late spring-summer. Boggy meadows, freshwater marshes; of conservation concern; 0-300 m; Calif.; c Mexico; Central America (Guatemala). Arenaria paludicola is federally listed as endangered, and now is known only from a few sites in San Luis Obispo County; urban development and resultant habitat conversion have impacted it significantly. Historical collections of A. paludicola are known from other areas of the California coast and from Washington.
Arenaria paludicola occurs also in Mexico, where it grows (and is sometimes confused) with A. bourgaei Hemsley in lakes and wet areas above 2000 meters; the relationship of these species to each other and to A. lanuginosa requires study.