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.
This herbaceous flowering perennial plant grows to 100150 cm (3959 in), or a rare 2 m (6 ft 7 in) tall, with erect leaves up to 90 cm (35 in) long and 3 cm (1.2 in) broad. The flowers are bright yellow, 710 cm (2.83.9 in) across, with the typical iris form. The fruit is a dry capsule 47 cm (1.62.8 in) long, containing numerous pale brown seeds.
I. pseudacorus grows best in very wet conditions, and is common in wetlands, where it tolerates submersion, low pH, and anoxic soils. The plant spreads quickly, by both rhizome and water-dispersed seed. It fills a similar niche to that of Typha and often grows with it, though usually in shallower water. While it is primarily an aquatic or marginal plant, the rhizomes can survive prolonged dry conditions.