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, as they have a prominently veined mid-rib and sword-like shape. However, the two plants are not closely related.

Acorus calamus is a species of flowering plant with psychoactive chemicals. It is a tall wetland monocot of the family Acoraceae, in the genus Acorus. Although used in traditional medicine over centuries to treat digestive disorders and pain, there is no clinical evidence for its safety or efficacy – and ingested calamus may be toxic – leading to its commercial ban in the United States.