Mat-forming herb with extensive prostrate (or floating) stems rooting at every node and with erect pedicels 2–5(10) cm from the nodes. Stems, petioles and leaves glabrous or with a few white hairs at the leaf sheaths. Lateral shoots developing from axial buds within the leaf sheaths.

Leaves alternate, long-stalked, petioles usually (0.5)1–3(10) cm. Blades 0.3–0.6 × 0.4–1.0 cm, normally broader than long, palmately dissected to more than half their length into three main lobes. Leaf lobes mostly narrow, 1–2 mm broad, the lateral lobes often divided again making the blades 5-lobed. Usually a mixture of leaves that are deeply dissected with narrow lobes and leaves with broader lobes, but the narrow-lobed leaves predominate.

Single flowers in leaf axis, usually terminally on the shoot.

Flower radially symmetric with 3 sepals and petals. Sepals 2.5–3 × 2–3 mm, about as long as or slightly shorter than petals and often broader than long, deflexed in mature stage, boat-shaped, glabrous, outer surface purple and inner surface yellow to purple. Petals 2–3 × 1–1.5 mm, obovate, often slightly notched, patent, yellow. Stamens 6–10, with yellow anthers and greenish yellow filaments. Receptacle 1 mm tall, glabrous. Carpels numerous, free.

The fruits are nutlets, glabrous, and with shorts beaks curved 90−120°. Heads of nutlets semiglobose, mostly 2–4 mm.

Sexual reproduction by seeds; extensive vegetative clonal growth by rooting stems. Flowers regularly and produces numerous nutlets which are assumed to ripen regularly. Germination of 40 % (Alsos et al. 2103).

Dispersal assumed to be mainly by water and by birds (internal and external) of both fruits and shoot fragments.

The two most small-grown species of Ranunculus in Svalbard – R. hyperboreus and R. pygmaeus – are often confused. They are easily distinguished by R. hyperboreus having 3 sepals and petals and a prostrate growth form with rooting shoots, whereas R. pygmaeus has 5 sepals and petals and grows as solitary plants with no rooting shoots. They also differ ecologically: Ranunculus hyperboreus is the perhaps only true aquatic vascular plant in Spitsbergen, whereas R. pygmaeus is confined to dry or only occasionally moist ground.

The ssp. arnellii (Svalbard except for Bjørnøya) and ssp. hyperboreus (on Bjørnøya) are distinguished by the following characters (mainly from Nilsson 2001): Ssp. arnellii is smaller, more slender, and more richly branched, leaf blades are darker green and more deeply divided, leaf lobes are only slightly divergent, the mid-lobe is obovate or narrowly oblong and often divided again, lateral lobes almost straight and divided again, and the beaks of the nutlets are bending to ca. 120°; ssp. hyperboreus is more coarse, less slender and often less richly branched, leaf blades are pure green and divided only to about the middle, leaf lobes are strongly divergent, the mid-lobe is ± elliptic-oblong and entire, the lateral lobes are often bending forwards and are entire or occasionally divided again, and the beaks of the nutlets are bending ca. 180° when ripe. Other characters reported to distinguish the taxa, based on material from other regions, are smaller flowers (3–5 mm vs. 4–7 mm), and smaller heads of nutlets (2–3 mm vs. 3–5 mm), see Nilsson (2001). These characters are not very diagnostic in the Svalbard material.

In small ponds or very wet moss tundra. The only vascular plant found in most aquatic site types. Probably indifferent as to soil reaction (pH).

In all bioclimatic zones and sections.

The plants in Svalbard except Bjørnøya conform to ssp. arnellii as described by Nilsson (2001) but the length of the petals relatively to sepals is variable. Subspecies arnellii is reported to be mainly Eurasian, present in Svalbard, arctic European Russia and Siberia, but extending across the Bering Strait to Alaska. It is not reported from Greenland or NE North America. Subspecies hyperboreus is reported to be distributed in NW Europe, to be the only race in Greenland, and to be the predominant race in North America (Elven et al. 2011). However, a superficial survey of just a few Greenland sheets (R. Elven observ. 2015) proved ssp. arnellii to be present in both E and W Greenland and to be sympatric with ssp. hyperboreum in these areas. This matter then needs to be re-investigated, and the rank of the two to be re-evaluated (in view of the Greenland sympatry which might suggest two independent species rather than subspecies). We are also in doubt whether it is justified to assign the Spitsbergen and Bjørnøya plants deserve to two different subspecies (or species).