The Sei whale, Balaenoptera borealis, is a dark-gray, stream-lined baleen whale that is found worldwide except in polar waters. It swims in small pods of 3-5 whales but larger groups may form at rich feeding grounds. It has very fine grey-black baleen that traps very small particles of food. It is a rorqual whale (a large baleen whale) that is similar to Bryde's whale.

The Sei whale is also called the Sardine whale, the Pollack whale, the Coalfish whale, the Japan Finner, and Rudolphi's Rorqual. Large numbers of these whales were hunted until recently for their oil and meat. It is the fastest of the great whales and can swim at about 23 mph (20 knots) in short bursts.

Sei whales grow to be about 39 to 52 feet (12-16 m) long and weigh about 20-30 tons. The females are slightly larger than the males, as with all baleen whales. Calves weigh about 1,600 pounds (725 kg) at birth!

Sei whales are carnivores that filter-feed plankton (tiny crustaceans like krill, copepods, pteropods, etc.) and small fish from the water