Ranunculus /ræˈnʌŋkjʊləs/ is a large genus of about 1750 species of flowering plants in the family Ranunculaceae. Members of the genus are known as buttercups, spearworts and water crowfoots.

The genus is distributed worldwide, primarily in temperate and montane regions. The familiar and widespread buttercup of gardens throughout Northern Europe (and introduced elsewhere) is the creeping buttercup Ranunculus repens, which has extremely tough and tenacious roots. Two other species are also widespread, the bulbous buttercup Ranunculus bulbosus and the much taller meadow buttercup Ranunculus acris. In ornamental gardens, all three are often regarded as weeds.

Buttercups usually flower in the spring, but flowers may be found throughout the summer, especially where the plants are growing as opportunistic colonizers, as in the case of garden weeds.

The water crowfoots (Ranunculus subgenus Batrachium), which grow in still or running water, are sometimes treated in a separate genus Batrachium (from Greek βάτραχος bátrakhos, "frog"). They have two different leaf types, thread-like leaves underwater and broader floating leaves. In some species, such as R. aquatilis, a third, intermediate leaf type occurs.

Ranunculus species are used as food by the larvae of some Lepidoptera species including the Hebrew character and small angle shades. Some species are popular ornamental flowers in horticulture, with many cultivars selected for large and brightly coloured flowers.

Buttercups are found in both hemispheres on all continents aside from Antarctica, and are primarily found in temperate or montane habitats. They likely originated in northern Eurasia during the late Eocene or Oligocene and rapidly radiated up to the present, dispersing worldwide. Fossil evidence suggests that despite no longer occurring there, they inhabited Antarctica up to the mid-late Pliocene, even while glaciations were rapidly altering the landscape.

Ranunculus gailensis and Ranunculus tanaiticus seed fossils have been described from the Pliocene Borsoni Formation in the Rhön Mountains, central Germany. Achenes labelled Ranunculus cf. tachiroei is known from the Pliocene of the Hengduan Mountains of China. Indeterminate achenes have been found from Neogene strata from the Meyer Desert Formation biota in the Transantarctic Mountains, which appear to have inhabited a periglacial environment. The oldest potential fossil is from the Late Eocene (initially identified as Miocene) Florissant Formation of Colorado, identified by Theodore Dru Alison Cockerell in 1922.

Buttercups are mostly perennial, but occasionally annual or biennial, herbaceous, aquatic or terrestrial plants, often with leaves in a rosette at the base of the stem. In many perennial species runners are sent out that will develop new plants with roots and rosettes at the distanced nodes.^{[citation needed]}