Ranunculus pinguis is a dark green, fleshy-leaved buttercup with relatively large, short-stalked flowers and narrow stiff yellow petals that grows in tufts.

It is an endemic species of New Zealand on the Auckland and Campbell Islands that flowers from December to January and sets seeds between February and April.

The leaves are stalked, with fleshy, glabrous or hairy blades of 2–8 cm that are narrowly diamond to kidney shaped, and shallowly incised to form three to five, or sometimes seven lobes, and a margin that is often with few large rounded teeth (or crenate).

On the Auckland Islands it is common among rocks, but also fully exposed along the ridgeline in moist stony soil and along the higher limit of the grasslands on peat.

Auckland specimens often have somewhat narrowly orbicular hairless leaves, and more numerous and larger petals that regularly carry three nectarines.