Plants often reddish tinged, heterophyllous, subsucculent. Stems prostrate to ascending, branching from base, not wiry, 15-40 cm. Leaves: ocrea 4-6 mm, proximal part funnelform, distal part silvery hyaline, soon disintegrating, leaving almost no fibrous remains; petiole 2-5 mm; blade often reddish tinged, elliptic to obovate or oblanceolate; 20-35 × 9-16 mm, margins flat, apex rounded; stem leaves (1.3-)2-2.6(-3.5) times as long as branch leaves; distal leaves overtopping flowers in distal part of inflorescence. Inflorescences axillary; cymes in most leaf axils, 1-4-flowered. Pedicels mostly exserted from ocreae, 2-4 mm. Flowers semi-open; perianth 3-3.5(-4) mm; tube 18-25% of perianth length; tepals overlapping, green, margins white or pink, petaloid, not keeled, broadly rounded, cucullate; midveins unbranched; stamens 8. Achenes exserted from perianth, brown, ovate, 3-gonous, 2.8-3.4(-4) mm, faces subequal or evidently unequal, apex not beaked, edges straight, shiny, minutely roughened; late-season achenes uncommon, 4.5-5 mm. 2n = 60.

Flowering Apr-Oct. Coastal salt and brackish marshes, swamps; of conservation concern; 0-10 m; Calif.

The origin and taxonomic affinities of Polygonum marinense are uncertain. T. R. Mertens and P. H. Raven (1965) suggested a relationship with P. oxyspermum C. A. Meyer & Bunge or the Mediterranean P. robertii Loiseleur-Deslongchamps. Polygonum marinense may be confused with P. ramosissimum. It can be distinguished by its subsucculent texture, funnelform ocreae, leaves rounded at the apices, and semi-open flowers. Marin knotweed is known from fewer than 15 locations in Marin, Napa, Solano, and Sonoma counties; it is threatened by coastal development.