Citrullus lanatus is a prostrate or climbing annual with several herbaceous, rather firm and stout stems up to 3 m long; the young parts are densely woolly with yellowish to brownish hairs while the older parts become hairless. The leaves are herbaceous but rigid, becoming rough on both sides; 60-200 mm long and 40-150 mm broad, ± ovate in outline, sometimes unlobed and ± entire, but usually deeply 3-lobed with the segments again lobed or doubly lobed; the central lobe is much the largest. The leaf stalks are somewhat hairy and up to 150 mm long. The tendrils are rather robust and usually divided in the upper part.
Male and female flowers occur on the same plant (monoecious) with the flower stalk up to 40 mm long and hairy. The receptacle is up to 4 mm long, broadly campanulate and hairy, the lobes are ± as long as the tube. The corolla is usually ± green or green-veined outside and white to pale or bright yellow inside and up to 30 mm in diameter. In southern Africa the flowering time of C. lanatus is mostly from January to April and the fruiting time mostly from February to May. Dry or rainy years will influence flowering and fruiting.
The fruit in the wild form is subglobose, indehiscent and up to 200 mm in diameter; the fruit stalk is up to 50 mm long. The fruits in the cultivated forms are globose to ellipsoid or oblong and up to 600 mm long and 300 mm in diameter. The rind in the ripe fruit is hairless and smooth, hard but not woody. In the wild forms the rind is pale or grey-green, usually mottled with irregular longitudinal bands of dark green or grey-green. In cultivated forms the rind is often concolorous yellowish to pale or dark green, or mottled with darker green, or marbled with a darker shade. The flesh in the wild form and some cultivated forms (citron watermelon) is firm and rather hard, white, green-white or yellowish. In cultivated forms the flesh is somewhat spongy in texture but very juicy and soft, pink to bright red-pink. The seeds are numerous, ± ovate in outline, sometimes bordered; in wild forms they are usually black or dark brown; in cultivated forms they are also white or mottled, mostly 6-12 mm long.