Iris pseudacorus, the yellow flag, yellow iris, or water flag, is a species of flowering plant in the family Iridaceae. It is native to Europe, western Asia and northwest Africa. Its specific epithet pseudacorus means "false acorus", referring to the similarity of its leaves to those of Acorus calamus (sweet flag), as they have a prominently veined mid-rib and sword-like shape. However, the two plants are not closely related. The flower is commonly attributed with the fleur-de-lis.
The plant was rated in second place for per day nectar production per flower in a UK plants survey conducted by the AgriLand project, which is supported by the UK Insect Pollinators Initiative. However, when number of flowers per floral unit, flower abundance, and phenology were taken into account, it dropped out of the top 10 for most nectar per unit cover per year, as did all plants that placed in the top ten, with the exception of common comfrey, Symphytum officinale.
