The ayu (アユ, 鮎, 年魚, 香魚?) or sweetfish, Plecoglossus altivelis, is an amphidromous fish, the only species in the genus Plecoglossus and family Plecoglossidae. It is a relative of the smelts, and is placed in the order Osmeriformes. Native to the Palearctic ecozone, it occurs in rivers, lakes, and coastal waters of western Hokkaidō in Japan southward to the Korean Peninsula, China, Taiwan, and Hong Kong.
The name “sweetfish” is due to the sweetness of its flesh. In reference to its typical one-year lifespan, it is also written as 年魚 (“year-fish”). The ayu is the prefectural fish of Gunma Prefecture and Gifu Prefecture.