Watermelon can be both the fruit and the plant of a vine-like (scrambler and trailer) plant originally from southern Africa, and is one of the most common types of melon. This flowering plant produces a special type of fruit known by botanists as a pepo, a berry 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, yellow and sometimes white) and a juicy, sweet interior flesh (usually pink, but sometimes orange, yellow, red and sometimes green if not ripe). It is also commonly used to make a variety of salads, most notably fruit salad.

A watermelon contains about 6% sugar and 92% water by weight. As with many other fruits, it is a source of vitamin C.

Notable is the inner rind of the watermelon, which is usually a light green or white color. This area is edible and contains many hidden nutrients that most people avoid eating due to its unappealing flavor.

The amino acid citrulline was first extracted from watermelon and analysed. Watermelons contain a significant amount of citrulline and after consumption of several kg, an elevated concentration is measured in the blood plasma; this could be mistaken for citrullinaemia or other urea cycle disorders.

Watermelon rinds are also edible, and sometimes used as a vegetable. In China, they are stir-fried, stewed or more often pickled. When stir-fried, the de-skinned and de-fruited rind is cooked with olive oil, garlic, chili peppers, scallions, sugar and rum. Pickled watermelon rind is also commonly consumed in the Southern US. Watermelon juice can also be made into wine.

Watermelon is also mildly diuretic.

Watermelons contain large amounts of beta carotene.

Watermelon with red flesh is a significant source of lycopene