Females are usually slightly larger than males and an exceptional individual may be up to 18.9 m in length and weigh 48 metric tons. For their length, humpbacks tend to be greater in girth than the other balaenopterid whales.
Coloration is black overall with irregular white markings on the throat, sides, and abdomen.
In some individuals the belly may be entirely white, or there may be white patterns dorsally. The flippers typically are white below but range from black to patterns of black and white dorsally, or even entirely white. The tail flukes are broad, serrated on the free edge, and black above with black and white coloration ventrally. Distinctive tail flukes may serve to identify individual humpback whales in many cases. As with other balaenopterid whales, humpbacks have a dorsal fin, which may be up to 31 cm in height, and is falcate to rounded in profile.