Rumex albescens is a subshrub with decumbent stems that grows from 30 to 200 cm long, varying from glabrous to moderately pilose. The leaves are simple and alternate, with lanceolate to narrowly deltate blades that measure 11–33 cm long and 2.8–11 cm wide. The leaf apex is acuminate to long-acuminate, and the base is cordate, with the base of upper leaves sometimes being broadly cuneate, especially in Nihoa plants. The upper surfaces of the leaves are typically glabrous or rarely sparsely short-pilose, mainly along the veins. In contrast, the lower surfaces can be sparsely to moderately pilose or glabrous in some O’ahu and Nihoa plants. The leaf margins are usually undulate, crisped, and erose-denticulate, though they can be subentire and not crisped in Nihoa plants. The petioles are 1.5–9 cm long, and the stipules form a sheath or ocrea. The flowers are numerous, forming diffuse, green or yellowish-green, many-branched, paniculate inflorescences that are 15–50 cm long. They are monoecious, with unisexual flowers, and the proportions of each sex can vary from nearly equal to one type strongly dominating. The flowers are actinomorphic (radially symmetric), with six tepals, the outer 3 being much smaller than the inner ones. In staminate flowers, the outer tepals are green and 2–2.5 mm long, with yellowish-green, membranous valves that are 3–3.5 mm long. In pistillate flowers, the outer tepals are green and 0.8–1 mm long, with yellowish-green, membranous valves that are 1.2–1.5 mm long, enlarging to 5–7 mm long and becoming cordate in fruit, with conspicuous venation and faces without tubercles. The stamens in staminate flowers are 6 and 1.8–2 mm long, with dithecal anthers that open by longitudinal slits. The ovary is superior and 1-celled, with one ovule and three styles, and the stigmas are fimbriate and dry. The fruit is a trigonous nut, enclosed by the valves, pale brown, 2–2.4 mm long, usually slightly wider. The sides of a single arm of a mature nut are subparallel, with the angle very narrowly acute and the margins of the arms without a conspicuous thickened rim. The seeds have well-developed, starchy, and oily endosperm. It is found in mesic forests, cliff faces, and dry shrublands between 180 – 1,220 meters.