Water-pepper or Water pepper (Persicaria hydropiper/ Polygonum hydropiper) is a plant of the family Polygonaceae.

It grows in damp places and shallow water. It is a cosmopolitan plant, found in Australia, New Zealand, temperate Asia, Europe, and North America. It has some use as a spice because of its pungent flavor.

The plant’s leaves from the cultivar are used as a vegetable. The wild type is very pungent.

Wild water pepper produces oils that cause skin irritation, and the acids in its tissues make the plant unpalatable to livestock.

The seeds of the water-pepper may be added to wasabi.