Watermelon (Citrullus lanatus (Thunb.) Matsum & Nakai, family Cucurbitaceae) refers to both fruit and plant of a vine-like (climber and trailer) herb originally from southern Africa and one of the most common types of melon. This flowering plant produces a special type of fruit known by botanists as a pepo, which has a thick rind (exocarp) and fleshy center (mesocarp and endocarp); pepos are derived from an inferior ovary and are characteristic of the Cucurbitaceae.

The watermelon fruit, loosely considered a type of melon (although not in the genus Cucumis), has a smooth exterior rind (green and yellow) and a juicy, sweet, usually red or yellow, but sometimes orange, interior flesh. The flesh consists of highly developed placental tissue within the fruit. The former name Citrullus vulgaris (vulgaris meaning "common" — Shosteck, 1974), is now a synonym of the accepted scientific name for watermelon, Citrullus lanatus.