The largest known Sei whale measured 20 meters in length, although most whales are between 12.2 and 15.2 meters long. Of this length, the head and body make up about 13 meters. Males are slightly smaller than females. Sei whales have a relatively slender body with a compressed tail stock that abruptly joins the flukes. The snout is pointed, and the pectoral fins are short. The dorsal fin is sickle shaped and ranges in height from 25 to 61 centimeters.

The body is typically a dark steel gray with irregular white markings ventrally. The ventrum has 38-56 deeps grooves, which may have some feeding function. Each side of the upper part of the mouth contains 300 - 380 ashy-black baleen plates. The fine inner bristles of these plates are whitish.

Average mass: 2e+07 g.