Nuphar is a genus of aquatic plants in the family Nymphaeaceae, with a temperate to subarctic Northern Hemisphere distribution. Common names include water-lily (Eurasian species; shared with many other genera in the same family), pond-lily, alligator-bonnet or bonnet lily, and spatterdock (North American species).

Nuphar species are aquatic, perennial, heterophyllous herbs growing from branching rhizomes, which are not stoloniferous. The rhizomes bear conspicuous leaf scars. The adventitious roots grow underneath and at the side of the rhizome. The leaves can be submerged, floating, or emergent. Their lamina can be ovate, elliptic, orbicular, linear, obovate, or lanceolate. The lamina has an entire margin, but it can be crisped in submerged leaves. The long, flattened, winged, or cylindrical petioles can be pubescent or glabrous.

The flowers are solitary, subglobose, diurnal, pedunculate, floating or emergent, with glabrous to pubescent peduncles. The flowers usually have 5–9 (but up to 12) persistent sepals, which are yellow, green or red and oblong, obovate or orbiculate. The numerous, oblong to spathulate petals are inconspicuous and hidden by the sepals. The yellow or red stamens with strap-like filaments are recurved at dehiscence. The monocolpate pollen grains are 40–71 μm long and 30.4–60 μm wide. The gynoecium consists of 4–36 fused carpels. The fruit is 0.5–5 centimetres (1⁄4–2 in) wide, ovoid to urceolate, borne on straight peduncles, and bears smooth, exarillate seeds up to 6 millimetres (1⁄4 in) long.

The genus Nuphar Sm. was published by James Edward Smith in 1809. The type species is Nuphar lutea (L.) Sm. The genus Nuphar Sm. has several synonyms: Nymphona Bubani published by Pietro Bubani in 1901, Nenuphar Link published by Johann Heinrich Friedrich Link in 1822, Nymphozanthus Rich. published by Louis Claude Marie Richard in 1808, and Ropalon Raf. published by Constantine Samuel Rafinesque in 1837. The genus has been divided into two sections: the autonymous section Nuphar sect. Nuphar with Nuphar lutea (L.) Sm. as the type species, as well as the section Nuphar sect. Astylus Padgett published by Donald Jay Padgett in 1999 with Nuphar advena (Aiton) W.T.Aiton as the type species.

The number of species in the genus is still under review. Until the mid-20th century, some botanists treated the genus as just a single variable species (for which the European N. lutea has priority), while some other authorities accepted about a dozen more species on the basis of traditional taxonomic standards. Recent molecular work has shown that there are substantial differences between the Eurasian species (sect. Nuphar) and American species (sect. Astylus), except for North American N. microphylla which clusters with the Eurasian species. Molecular taxonomy has shown conclusively that recognition of so few species is out of the question, and forced an increased number of recognised species; some sources list about seventy. The Kew Gardens plant list includes over twenty accepted species, subspecies and varieties; it also has a similar number as yet unresolved, together with over twenty synonyms.

Nuphar sect. Astylus Padgett

Nuphar sect. Nuphar (autonym)

There also are several interspecific hybrids: