Masago is capelin roe, commonly used as a sushi garnish.
Capelin is a small ocean fish in the smelt family, and its tiny eggs are harvested, seasoned, and sold as masago. The roe is usually pale orange, although food coloring and flavorings can produce brighter shades. Its small size and lightly crunchy texture make it useful as a topping for rolls, gunkan-maki, and other sushi presentations.
Masago is often confused with tobiko. Tobiko is flying-fish roe and generally has a larger, firmer pop, while masago is smaller and softer. The two may look similar, and restaurants sometimes use masago as a less expensive substitute for tobiko. Ikura is different again: it is salmon roe, with much larger orange-red eggs.
The Japanese word “masago” is also associated with fine sand, a reference to the roe’s tiny grains. In ingredient lists, masago may be described broadly as smelt roe, because capelin belongs to the smelt family. That broader wording does not change the standard sushi identification: capelin roe is masago.