The blue whale is a marine mammal belonging to the baleen whales. At 30 metres (98 ft) in length and 180 tonnes or more in weight, it is the largest extant animal and is the heaviest known to have existed. As with other baleen whales, its diet consists almost exclusively of small crustaceans known as krill.

Blue whales were abundant in nearly all the oceans on Earth until the beginning of the twentieth century. For over a century, they were hunted almost to extinction by whalers until protected by the international community in 1966. Before whaling, the largest population was in the Antarctic, numbering approximately 239,000. A 2002 report estimated there were 5,000 to 12,000 blue whales worldwide.

Blue whales are rorquals (family Balaenopteridae), a family that includes the humpback whale, the fin whale, Bryde's whale, the sei whale, and the minke whale. The blue whale has a long tapering body that appears stretched in comparison with the stockier build of other whales. The head is flat, U-shaped and has a prominent ridge running from the blowhole to the top of the upper lip. The front part of the mouth is thick with baleen plates; around 300 plates, each around one metre (3 ft) long, hang from the upper jaw.

An adult blue whale can eat up to 40 million krill in a day. The whales always feed in the areas with the highest concentration of krill, sometimes eating up to 3,600 kilograms (7,900 lb) of krill in a single day. The daily energy requirement of an adult blue whale is in the region of 1.5 million kilocalories. Their feeding habits are seasonal. Blue whales gorge on krill in the rich waters of the Antarctic before migrating to their breeding grounds in the warmer, less-rich waters nearer the equator. The blue whale also incidentally consumes small fish, crustaceans and squid caught up with krill.

Females typically give birth once every two to three years at the start of the winter after a gestation period of 10 to 12 months. The calf weighs about 2.5 tonnes and is around 7 metres (23 ft) in length. Blue whale calves drink 380-570 litres (100-150 U.S. gallons) of milk a day.

Sexual maturity is typically reached at five to ten years of age. Scientists estimate that blue whales can live for at least 80 years.

Blue whales are not easy to catch or kill. Their speed and power meant that they were rarely pursued by early whalers, who instead targeted sperm and right whales. In 1864, the Norwegian Svend Foyn equipped a steamboat with harpoons specifically designed for catching large whales. Although it was initially cumbersome and had a low success rate, Foyn perfected the harpoon gun, and soon several whaling stations were established on the coast of Finnmark in northern Norway. Soon, blue whales were being hunted off Iceland (1883), the Faroe Islands (1894), Newfoundland (1898), and Spitsbergen (1903). Blue whale hunting was banned in 1966 by the International Whaling Commission, and illegal whaling by the Soviet Union finally halted in the 1970s, by which time 330,000 blue whales had been caught in the Antarctic, 33,000 in the rest of the Southern Hemisphere, 8,200 in the North Pacific, and 7,000 in the North Atlantic. The largest original population, in the Antarctic, had been reduced to 0.15% of their initial numbers.

The IUCN Red List counts the blue whale as "endangered" as it has since the list's inception. In the United States, the National Marine Fisheries Service lists them as endangered under the Endangered Species Act. Due to their enormous size, power and speed, adult blue whales have virtually no natural predators. There is one documented case in National Geographic Magazine of a blue whale being attacked by orcas off the Baja California Peninsula; although the orcas were unable to kill the animal outright during their attack, the blue whale sustained serious wounds and probably died as a result of them shortly after the attack.