The green horseradish-like paste commonly served with sushi is wasabi. True wasabi comes from the grated rhizome of Eutrema japonicum, a Japanese plant in the mustard and cabbage family, Brassicaceae.
Wasabi grows naturally along cool mountain stream beds and is difficult and expensive to cultivate at scale. Fresh rhizome is traditionally grated into a paste, often using a sharkskin grater. Its pungency comes largely from allyl isothiocyanate, the same volatile compound associated with horseradish and mustard. This creates a sensation that rises into the nose rather than producing the mouth-burning heat of chili peppers.
A frequent misconception is that all sushi-restaurant wasabi is genuine wasabi. Outside Japan, much of the green paste sold as wasabi is a substitute made primarily from horseradish, with mustard, starch, and green coloring. Fresh wasabi also loses flavor quickly after grating, so chefs may prepare it immediately before serving. Sushi chefs often place it between the fish and rice to protect its aroma.