Tansy (Tanacetum vulgare) is a perennial, herbaceous flowering plant in the genus Tanacetum in the aster family, native to temperate Europe and Asia. It has been introduced to other parts of the world, including North America, and in some areas has become invasive. It is also known as common tansy, bitter buttons, cow bitter, or golden buttons. The Latin word vulgare means "common".

Tansy is a flowering herbaceous plant with finely divided compound leaves and yellow, button-like flowers. It has a stout, somewhat reddish, erect stem, usually smooth, 50–150 cm (20–59 in) tall, and branching near the top. The leaves are alternate, 10–15 cm (3.9–5.9 in) long and are pinnately lobed, divided almost to the center into about seven pairs of segments, or lobes, which are again divided into smaller lobes having saw-toothed edges, giving the leaf a somewhat fern-like appearance. The roundish, flat-topped, button-like, yellow flower heads are produced in terminal clusters from mid-to-late summer. The scent is similar to that of camphor with hints of rosemary. The leaves and flowers are toxic if consumed in large quantities; the volatile oil contains toxic compounds including thujone, which can cause convulsions and liver and brain damage. Some insects, notably the tansy beetle Chrysolina graminis, have resistance to the toxins and subsist almost exclusively on the plant.

Tansy is native to Eurasia; it is found in almost all parts of mainland Europe, as well as Britain and Ireland. It is absent from Siberia and from some of the Mediterranean islands. The ancient Greeks may have been the first to cultivate it as a medicinal herb. In the sixteenth century it was considered to be "necessary for a garden" in England.

Tansy has a long history of use. The first historical records of its cultivation for medicinal purposes involved the ancient Greeks. In the 8th century AD it was grown in the herb gardens of Charlemagne and by Benedictine monks of the Swiss monastery of Saint Gall. Tansy was used to treat intestinal worms, rheumatism, digestive problems, fevers, sores, and to bring out measles.

During the Middle Ages and later, high doses were used to induce abortions. On the other hand, tansy was also used to help women conceive and to prevent miscarriages. In the 15th century, Christians began serving tansy with Lenten meals to commemorate the Passover bitter herbs eaten by the Israelites. Tansy was thought to have the added Lenten benefits of controlling flatulence brought on by days of eating fish and pulses and of preventing the intestinal worms believed to be caused by eating fish during Lent.

Tansy was used as a face wash and was reported to lighten and purify the skin. In the 19th century, Irish folklore suggested that bathing in a solution of tansy and salts would cure joint pain.

Tansy has also been used as insect repellent and in embalming. It was packed into coffins, wrapped in funeral winding sheets, and tansy wreaths were sometimes placed on the dead. Henry Dunster, the first president of Harvard University, was buried wearing a tansy wreath in a coffin packed with tansy; when the cemetery was moved in 1846 the tansy had maintained its shape and fragrance, helping to identify the president's remains. By the 19th century, tansy was used so much at New England funerals that people began to disdain it for its morbid association with death.

During the American colonial period, meat was frequently rubbed with or packed in tansy leaves to repel insects and delay spoilage. Tansy was frequently worn at that time in shoes to prevent malaria and other fevers; it has been shown, however, that some mosquito species, including Culex pipiens, take nectar from tansy flowers.