Alopecurus geniculatus is a species of grass known by the common name water foxtail or marsh foxtail. It is native to much of Eurasia and introduced into North America, South America, and Australia. It grows in moist areas.
Alopecurus geniculatus is a perennial grass forming bunches of erect stems up to about 60 cm (24 inches) in height. The leaves are up 12 cm (5 inches) in length. The inflorescence is a dense panicle up to 6 or 7 cm long which blooms in dusty yellow-orange anthers.
Alopecurus geniculatus is a component of Purple moor grass and rush pastures a type of Biodiversity Action Plan habitat in the UK. It occurs on poorly drained neutral and acidic soils of the lowlands and upland fringe.