Rhizomes erect, unbranched. Leaf blade cordate-ovate to ovate-elliptic, 5--12 × 3.5--9 cm, papery, abaxially glabrous, scarcely peltate, base deeply cordate and basal lobes parallel to contiguous, margin entire. Flower floating, 3--6 cm in diam. Calyx insertion on receptacle prominently tetragonous; sepals broadly lanceolate to narrowly ovate, 2--3.5 cm, obscurely veined, persistent. Petals 8--15(--17), white, broadly lanceolate, oblong, or obovate, 2--2.5 cm, transition to stamens gradual. Filament of inner stamens wider than anther, connective apically unappendaged. Carpels completely united, walls between locules of ovary single. Stigma rays 5--8(--10); carpellary appendages ovate. Fruit globose, 2--2.5 cm in diam. Seeds ellipsoid, 2--3(--4) mm, smooth. Fl. Jun--Aug. 2n = 112.

The treatment of this species in E Asia is problematic. In North America and Europe Nymphaea tetragona is restricted to boreal regions above 50° N latitude, where the plants have a prominently tetragonous receptacle, flowers with a reddish to purplish center, and thinner leaves abaxially with raised veins. Plants in Korea, N Japan, and Russia, as depicted in Lee (Fl. Korea, 216. 1997) and Kadono (Aquatic Pl. Japan, 112. 1994), and observed on specimens, have these characteristics. Plants of N China and the Himalayas may belong here also, but to determine this will require further study of better material. However, plants of tropical and warm temperate China, Japan, and Vietnam appear to differ in having an only slightly tetragonous receptacle, flowers with a yellow center, and thicker leaves abaxially with impressed veins. Such plants are sometimes known in cultivation in Europe and North America as N. tetragona var. angusta Caspary, but were originally sent in 1805 by William Kerr from Guangdong Province and described under the name of Castalia pygmaea Salisbury ( N. pygmaea (Salisbury) W. T. Aiton). Further study may indicate that much of the Chinese and Japanese material should be segregated as N. pygmaea or at some other rank. One might expect hybridization between the two taxa to occur where they coexist, as with other related Nymphaea, and the level of sterility displayed by such hybrids should be examined as evidence of the degree of their relationship.

An examination of the type of Nymphaea esquirolii H. Léveillé & Vaniot from Guizhou Province indicates that it belongs with N. pygmaea . The assignment of N. crassifolia (Handel-Mazzetti) Nakai ( Castalia crassifolia Handel-Mazzetti; N. tetragona var. crassifolia (Handel-Mazzetti) Chu) to one of these two forms is unknown, as the type has not been examined. Nymphaea acutiloba de Candolle is of uncertain identity and lacks a type specimen. Although referred by Conard (Proc. Carnegie Inst. Wash. 4:170. 1905) to N. tetragona, the description cannot apply to this taxon and more closely matches N. nouchali or N. lotus .

Ponds, lakes; near sea level to 4000 m. Fujian, Guangdong, Guangxi, Guizhou, Hainan, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, Nei Mongol, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Xinjiang, Xizang, Yunnan, Zhejiang [India, Japan, Kashmir, Kazakhstan, Korea, Russia, Vietnam; North America, Europe].