The body is slim and streamlined and is rather more bulky than the fin whale, but still laterally compressed in the caudal region. The head is less pointed than that of the fin whale and less rounded than the blue whale. The head of the sei whale can be one fifth to one quarter of the animals body length. The relative size of the head increases with age.
The dorsal fin is 0.25 to 0.6 m high, and located a third of the way from the caudal notch to the tip of the rostrum. The width of the fluke is about one quarter of the total body length; their trailing edge is almost straight and there is a deep median notch in the flukes.
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The baleen of the sei whale is uniformly grayish black with fine white or gray fringes. Some whales have a few lighter plates towards the front of the mouth, and these can cause confusion with fin whales. The longest baleen plates measure 75-80 cm and the number of plates on each side of the jaw ranges from 300-400.
Sei whales are dark gray or bluish gray in color on their back and sides. Their belly is a region of grayish white that is almost always confined to the area of the ventral grooves. There is much individual variation though. Some individuals have a white or light area to the flippers and white streak may be seen extending downwards behind the ear. The flipper or the flukes are dark gray or bluish dorsally and ventrally. The body often has a galvanized appearance due to light colored scars inflicted by various types of parasites. The jaw, lips, and mouth cavity are all also gray.
Sei whales are generally 12-16 m long, weighing about 20 tons, although they can reach lengths of 18.6 m (in males) and 21.0 m (in females), and can weigh up to 40 tons. Females are generally about 0.5 m longer than the males.