Allium sphaerocephalon is a plant species in the Amaryllis family known as round-headed leek, round-headed garlic, ball-head onion, and other variations on these names. Drumstick allium is another common name applied to this species. Some publications use the alternate spelling Allium sphaerocephalum. It is a bulbous herbaceous perennial plant.

Allium sphaerocephalon is found in the wild across all parts of Europe except in the northern and western countries ( Scotland, Ireland, Netherlands, Scandinavia, and the Baltic States). Its native range extends to northern Africa and to western Asia as far east as Iran. It is also reportedly naturalised in New York State, US. In England it grows wild only in the Avon Gorge so is known locally as the Bristol onion.

The species is prized by gardeners because of its striking floral display. The spherical "head" (technically an umbel) is borne on a long scape, up to 50 cm in height, usually in July. It can contain hundreds of deep purple flowers.

The specific epithet sphaerocephalon derives from ancient Greek, meaning "spherical head".

This plant has gained the Royal Horticultural Society’s Award of Garden Merit.