The average length of male sei whales in the Atlantic Ocean is 13 m (40 ft.), but females are slightly larger. The longest sei whale yet measured, a female, was 20 m (65 ft.). The sei whale is a smaller relative of the fin whale.

In the western Atlantic, sei whales migrate from Florida to Labrador and leave northern waters before September. They also occur all summer long off Canada's west coast. They are normally found far off shore. They frequently feed on the surface, trapping schools of small fish or plankton in their mouths. Like many of the rorquals, sei whales feed in temperate and subpolar seas in summer, and migrate to subtropical areas in winter.

Although these relatively small, lean whales are less profitable for whalers than the much larger blue whale, they have been hunted since the 1860s. Although only very limited hunting is permitted, the sei whale populations have not experienced a resurgence; the species is rare and considered endangered. Sei whales are also preyed upon by killer whales.