Rumex skottsbergii is a shrub characterized by usually stiffly erect stems that are 70 to 100 cm long and glabrous. It has a taproot system. The leaves of this shrub are simple and alternate, with flat, ovate to oblong-ovate or oblong-elliptic blades measuring 5.5 to 13.5 cm in length and 1.4 to 5.8 cm in width. The leaf apex is acute to obtuse, and the base is broadly cuneate. The surfaces of the leaves are glabrous, thick, and somewhat glossy, with entire and flat margins. The petioles are 1.7 to 5 cm long, and the stipules form a sheath or ocrea. The numerous flowers are found in small to medium-sized, green or yellowish-green, somewhat congested, many-branched, usually erect, paniculate inflorescences measuring 10 to 20 cm long and 5 to 15 cm wide. It is monoecious with unisexual flowers, with the proportions of each sex varying. The tepals are in sets of six, with the outer three being much smaller than the inner ones, and connate at the base into a minute floral tube. The staminate flowers have about 1 mm long outer tepals, with the valves being membranous and greenish. The pistillate flowers have greenish outer tepals, approximately 1 to 1.5 mm long, enlarging to 5.5 to 6.2 mm in fruit and becoming cordate with conspicuous venation and denticulate margins. There are six stamens with dithecal anthers that open longitudinally. The superior ovary is 1-celled with one ovule, three styles, and fimbriate, dry stigmas. The fruit is a pale brown trigonous nut enclosed by the valves, about 1.8 to 2 mm long, with two sides of a single arm of a mature nut distinctly diverging at about a 45° angle. The seeds contain well-developed, starchy, and oily endosperm. It inhabits dry ʻaʻā and pāhoehoe lava, and possibly ash between 460-1,300 meters.