Alisma lanceolatum is a species of aquatic plant in the water plantain family known by the common names lanceleaf water plantain and narrow-leaved water plantain. It is widespread across Europe, North Africa and temperate Asia. It is naturalized in Australia, New Zealand, Oregon, California and British Columbia. It is considered a noxious weed in some places. This species is a weed of rice fields in many areas, including New South Wales and California. In England and Wales it is occasionally locally found, in Ireland it is rare, and Scotland it is very rare.