The tansy beetle is a species of leaf beetle. The common name derives from its main foodplant, Tansy, but it can also use other wetland plants such as Gypsywort and Water Mint. It measures 7.7–10.5 mm in length and has a characteristic bright metallic green colouration, with pitted elytra and a coppery tinge. In addition to the nominotypical subspecies, which repeats the specific name, C. graminis graminis, there are five further distinct subspecies of Tansy beetle, which, collectively, have a Palearctic distribution, although in the majority of countries where it is found the species is declining. In the United Kingdom it is designated as 'Nationally Rare'. The stronghold population here is located along the banks of the river Ouse in York, North Yorkshire. Other, small, fenland populations exist at Woodwalton Fen and at Welney Wildfowl and Wetlands Trust (WWT) reserve.

Tansy 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".