西瓜（学名：Citrullus lanatus），古称寒瓜，是葫芦科西瓜属的一种植物或其果实。原产于非洲，是一种双子叶开花植物，形状像藤蔓，叶子呈羽毛状。它所结出的果实是假果，且属于植物学家称为假浆果的一类。果实外皮光滑，呈绿色或黄色有花纹，果瓤多汁为红色或黄色。西瓜的产量十分丰富，差不多每一株藤就可以结出达100个果实。但西瓜种植时怕水，水浇多了之后容易导致甜度降低，水伤。

西瓜可分为野生或种植的。野生的西瓜称为野生西瓜种质（学名：Citrullus lanatus var. citroides），瓢为白色，而种植的西瓜则称为栽培西瓜品系（学名：Citrullus lanatus var. lanatus）。

Citrullus lanatus is a plant species in the family Cucurbitaceae, a vine-like (scrambler and trailer) flowering plant originally from West Africa. It is cultivated for its fruit. The subdivision of this species into two cultivars, watermelons (Citrullus lanatus (Thunb.) var. lanatus) and citron melons (Citrullus lanatus var. citroides (L. H. Bailey) Mansf.), originated with the erroneous synonymization of Citrullus lanatus (Thunb.) Matsum. & Nakai and Citrullus vulgaris Schrad. by L.H.Bailey in 1930. Molecular data including sequences from the original collection of Thunberg and other relevant type material, show that the sweet dessert watermelon (Citrullus vulgaris Schrad.) and the bitter wooly melon Citrullus lanatus (Thunb.) Matsum. & Nakai are not closely related to each other. Since 1930, thousands of papers have misapplied the name Citrullus lanatus (Thunb.) Matsum. & Nakai for the dessert watermelon, and a proposal has therefore been submitted to conserve the name with this meaning.