Tansy (Tanacetum vulgare) is a perennial herbaceous flowering plant of the aster family that is native to temperate Europe and Asia. It has been introduced to other parts of the world and in some cases has become invasive. It is also known as Common Tansy, Bitter Buttons, Cow Bitter, Mugwort, or Golden Buttons.

Tansy is a flowering herb with finely divided compound leaves and yellow, buttonlike flowers. It has a stout, somewhat reddish, erect stem, usually smooth, 50-150 cm tall, and branching near the top. The leaves are alternate, 10-15 cm 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, thus giving the leaf a somewhat fernlike appearance. The roundish, flat-topped, buttonlike, 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 said to be poisonous if consumed in large quantities. The plant's volatile oil is high in thujone, a substance found in absinthe that can cause convulsions. Some insects, notably the tansy beetle, have evolved resistance to tansy and live almost exclusively on it.

Tansy is native to Eurasia; it is found in almost all parts of mainland Europe. It is absent from Siberia and some of the Mediterranean islands. The ancient Greeks may have been the first to cultivate it as a medicinal herb. In about 1525, it was listed (by the spelling "Tansey") as "necessary for a garden" in Britain.

Tansy was formerly used as a flavouring for puddings and omelets, but that is almost unknown now. But they were certainly relished in days gone by, for Gerarde speaks of them as "pleasant in taste", and he recommends tansy sweetmeats as "an especial thing against the gout, if every day for a certain space a reasonable quantitie thereof be eaten fasting". According to liquor historian A. J. Baime's book Big Shots, bourbon magnate Jack Daniel enjoyed drinking his bourbon with sugar and crushed tansy leaf.