水蓼（学名：Polygonum hydropiper；同义词：Persicaria hydropiper）是一种蓼科蓼属植物，也称辣蓼。

一年生草本，高20～80厘米，直立或下部伏地。茎红褐色，直立多分枝，无毛，节常膨大，且具须根。全缘披针形的叶子互生，长4～9厘米，宽5～15毫米，两端均有腺点；紫褐色筒状的托叶鞘，有短缘毛和腺点。夏秋开淡绿色或淡红色花，花被4～5裂，卵形或长圆形，有黄色透明的腺点；穗状花序腋生或顶生，细弱下垂，下部的花间断不连。卵形瘦果，侧扁，一面平一面突起，表面有小点。

Water-pepper, water pepper or marshpepper knotweed (Persicaria hydropiper, syn. Polygonum hydropiper) is a plant of the family Polygonaceae. It grows in damp places and shallow water. It is a widespread plant, found in Australia, New Zealand, temperate Asia, Europe, and North America. It has some use as a spice because of its pungent flavor.