The sei whale is a cosmopolitan species, with a mainly offshore distribution, occurring in the North Atlantic, North Pacific and Southern Hemisphere, but probably not in the Northern Indian Ocean (Rice 1998); it is an occasional visitor to the Mediterranean (Reeves and Notarbartolo di Sciara 2006). Sei whales migrate between tropical and subtropical latitudes in winter and temperate and subpolar latitudes in summer, staying mainly in water temperatures of 8-–18°C, and tend not to penetrate to such high latitudes as other rorquals. Their winter distribution seems to be widely dispersed and is not fully mapped (Horwood 1987, 2002).

In the North Pacific, sei whales in summer are distributed mainly north of 40°N, including the Gulf of Alaska and Aleutian Islands (US), and to some extent into the Bering Sea, but not into the Okhotsk Sea. The wintering grounds are poorly known, but sei whales were formerly caught in winter off the Bonin Islands (Japan) (IWC 2006); animals tagged there have been recaptured throughout the summer range (Masaki 1977).

In the North Atlantic the summer distribution seems to be quite variable from year to year; however, sei whales typically occur north of an arc running from south of Nova Scotia in the west, to the northwestern British Isles and Trondheim (Norway) in the east. They occur as far north as the Davis Strait and the northern end of the Denmark Strait. Occasional incursions into other areas have been noted (e.g. the Gulf of Maine, Schilling et al. 1993). Peaks of catch rates in early and late summer suggested a northward and southward migration through the former Nova Scotia whaling ground (Mitchell 1975). Sei whales have been scarce in Norwegian waters in recent years despite significant catches there in the past. Sei whales were caught in limited numbers, mainly in late summer, off northwestern Spain (Aguilar and Sanpera 1982), and have been recorded in winter as far south as the Caribbean Sea and Cap Blanc, Mauritania (Rice 1998).

The summer (Jan–Feb) distribution in the southern hemisphere is mainly in the zone 40–50°S in the South Atlantic and southern Indian oceans, and 45–60°S in the South Pacific (Miyashita et al. 1995). Known wintering grounds include a number of former low–latitude whaling grounds, including northeastern Brazil at 7°S (da Rocha 1983), Peru at 6°S (Valdivia et al. 1982), and in earlier years off Angola and the Congo (IWC 2006). Catches off western South Africa (Donkergat) and eastern South Africa (Durban) showed peaks in spring and autumn, suggestive of populations on migration routes (Horwood 1987).

Chilean whaling records (32–38°S) show catches and sightings of “sei” whales throughout the year including summer until the early 1980s, but some of these may have been Bryde’s whales. However, some sei whales have been observed feeding in summer at around 42°S off Chile in recent years (Galletti et al. 2005).