Female humpbacks grow to an average of about 15 m in length and are a bit larger than males. The body is stocky and dark gray in color, the head is broad and flat, and the dorsal fin is a little more than two-thirds of the way back on the body and can vary in shape. Distinguishing features include fleshy knobs or tubercules on the head and lower lip, and long pectoral fins or flippers one-third the body length. Humpback whale flukes have an irregular trailing edge and black and white pigmentation on the nderside. This fluke pattern is often so distinctive that it can be used to identify individuals.

Humpback whales in the North Pacific are currently divided into four stocks: the coastal California/Oregon/Washington - Mexico stock, the Mexico offshore island stock (feeding destination unknown), the central North Pacific stock (Hawaii/Alaska), and the western North Pacific stock.