(Prionace glauca), abundant shark of the family Carcharhinidae found in all oceans, from warm temperate to tropical waters. Also known as the blue whaler, the blue shark is noted for its attractive, deep-blue colouring contrasting with a pure-white belly. It is a slim shark, with a pointed snout, saw-edged teeth, and long, slim pectoral fins. Maximum length is about 4 metres (12 feet).

Fish form the main part of its diet, but the blue shark is also a scavenger and sometimes follows ships for extended periods. It is noted for the speed with which it appears near slaughtered whales and its avidity in feeding on the carcasses. It is generally considered potentially dangerous to man and has been implicated in several unprovoked attacks.