Centaurea cyanus, commonly known as cornflower or bachelor's button (among other names), is an annual flowering plant in the family Asteraceae native to Europe. In the past, it often grew as a weed in cornfields (in the broad sense of "corn", referring to grains, such as wheat, barley, rye, or oats), hence its name. It is now endangered in its native habitat by agricultural intensification, particularly by over-use of herbicides. However, Centaurea cyanus is now also naturalised in many other parts of the world, including North America and parts of Australia through introduction as an ornamental plant in gardens and as a seed contaminant in crop seeds.

Centaurea cyanus is an annual plant growing to 20–100 centimetres (8–39 in) tall, with grey-green branched stems. The leaves are lanceolate and 3–10 cm (1–4 in) long. The flowers are most commonly an intense blue colour and arranged in flowerheads (capitula) of 1.5–3 cm diameter, with a ring of a few large, spreading ray florets surrounding a central cluster of disc florets. The blue pigment is protocyanin, which in roses is red. Fruits are approx. 3.5 mm long with 2–3 mm-long pappus bristles. It flowers all summer.

Centaurea cyanus is a diploid flower (2n = 24). The genetic diversity within populations is high, although there could be a future decline in diversity due to population fragmentation and intensive agriculture. In general, Centaurea cyanus is a self-incompatible species. However, selfing still occurs occasionally, but results in inbreeding depression.

Centaurea cyanus is native to temperate Europe, but is widely naturalized outside its native range.

It has been present in Britain and Ireland as an archaeophyte (ancient introduction) since the Iron Age. In the United Kingdom, it has declined from 264 sites to just 3 sites in the last 50 years.

In reaction to this, the conservation charity Plantlife named it as one of 101 species it would actively work to bring 'back from the brink'.

In the County Clare (VC H9) in Ireland, C. cyanus is recorded in arable fields as very rare and almost extinct, while in northeast Ireland, it was abundant before the 1930s.

Centaurea cyanus is considered a noxious weed in arable crops, especially cereals and rapeseed. In winter wheat, one plant per m² can cause a yield loss of up to 30 kg / ha. Centaurea cyanus produces around 800 seed per plant, which are either shed shortly before the harvest of cereals, or they are threshed together with the cereal grains, contributing to the further spread of the species by the harvesting machinery and contaminated seed. The occurrence of Centaurea cyanus strongly decreased during the last decades due to improved seed cleaning, more intensive nitrogen fertilization and herbicide use. However, Centaurea cyanus has become more common in cropland due to an increase in crop rotations dominated by winter cereals and rapeseed and the use of more selective herbicides with a low effectiveness against Centaurea cyanus. In addition, the emergence of resistance against the herbicide class of sulfonylureas has been reported recently. Due to its strong roots, Centaurea cyanus is difficult to control mechanically in spring.