紫萍（学名：Spirodela polyrhiza）又名水萍，为天南星科紫萍属下的一个种，过去曾被归类于浮萍属（Lemna）。

Spirodela polyrhiza (orth. var. S. polyrrhiza) is a species of duckweed known by the common names common duckmeat, greater duckweed, common duckweed, and duckmeat. It can be found nearly worldwide in many types of freshwater habitat. It is a perennial aquatic plant usually growing in dense colonies, forming a mat on the water surface. Each plant is a smooth, round, flat disc one half to one centimeter wide. It produces several minute roots. It also produces a pouch containing male and female flowers. The top part dies in the fall and the plant often overwinters as a turion.