Sei whales are the third largest whales on Earth, reaching lengths of up to 20 m. They are also one of the fastest. They can reach speeds in excess of 40 kph for short periods of time.

The dorsal fin is exceptionally tall for a baleen whale and located near the rear end of the whale about 2/3 down its back.

Sei whales are migratory and can be found in every ocean, although they prefer temperate and subtropical waters.

Sei whales are filter feeders like all other baleen whales. However, the sei whale is the only baleen whale that uses both skimming and gulping feeding techniques. Once its prey is captured, it then pushes the water out of its mouth while keeping its food trapped in the baleen plates. During feeding periods, it can dive for up to 20 minutes while searching for food.

Female sei whales bare a single offspring once every 2–3 years, and on rare occasions, twins may be born. Maturity is typically reached at around 5–14 years, which is when these whales can begin mating and reproducing. The average lifespan for sei whales is 50–60 years.