Sei whales have average adult body lengths of 15m (maximum 19.5m).
The blow is tall and vertical, although in windy conditions it may appear angled or bushier. The dorsal fin is located two-thirds of the way along the back and is tall and erect, rising steeply from the back.
Overall colouration appears a dark grey-brown in good light, with a pale-grey chevron marking extending from the flank and over the back behind the head. A pale, forward-sweeping “brush mark” pattern is also present on the mid-flank. The jawline is slightly arched and downturned at the tip.