大薸（学名：Pistia stratiotes），又名大萍、大蕊萍、大莲、水芙蓉，为天南星科大薸属的唯一物种。

多年生浮生草本，根须垂悬水中，是漂浮性水生植物。楔形叶片呈莲座状簇生在短缩茎上，叶肉疏松，浮生于水面。腋芽能生出葡萄茎，顶端长出新株。白色花，雌雄同株，肉穗花序。浆果内有绿黄色的种子。

Pistia is a genus of aquatic plant in the arum family, Araceae. The single species it comprises, Pistia stratiotes, is often called water cabbage, water lettuce, Nile cabbage, or shellflower. Its native distribution is uncertain, but probably pantropical; it was first discovered from the Nile near Lake Victoria in Africa. It is now present, either naturally or through human introduction, in nearly all tropical and subtropical fresh waterways. The genus name is derived from the Greek word πιστός (pistos), meaning "water," and refers to the aquatic nature of the plants.