Plants perennial, glabrous, with vertical rootstock. Stems usually procumbent, rarely ascending, usually producing axillary shoots below 1st-order inflorescence or at proximal nodes, 30-60 cm. Leaf blades narrowly linear-lanceolate, 6-17 × 1-3 cm, usually ca. 7-10 times as long as wide, widest near middle, usually thick, not coriaceous or subcoriaceous, base cuneate, margins entire, usually strongly undulate and/or crenulate, apex acute. Inflorescences terminal and axillary, terminal usually occupying distal 1/ 1/ 3 of stem, rather dense towards apex, distinctly interrupted in proximal 1/ 2, usually broadly paniculate (branches almost at right angles to main axis, simple or with few 2d-order branches). Pedicels articulated in proximal 1/ 3, filiform, 4-7 mm, not more than 2-2.5 times as long as inner tepals, articulation slightly swollen. Flowers 10-20 in whorls; inner tepals, broadly deltoid or deltoid-ovate, 3-4 × 3.2-4(-4.5) mm, base truncate, margins entire or indistinctly crenulate, apex obtuse or subacute; tubercles absent, rarely small and indistinct. Achenes brown or dark reddish brown, 2-3 × 1.5-2 mm. 2n = 20. Flowering early summer. Mostly coastal and alluvial habitats: sea beaches, shores of rivers and streams, wet meadows; 0-200 m; Nunavut., Que. Some specimens of Rumex subarcticus have well-developed tubercles similar to those of R. pallidus (N. M. Sarkar 1958), to which it is closely related and of which it may be regarded as a northwestern subspecies or variety (see Á. Löve 1986).