Its body is typical of the Pacific salmon group (see Salmons, Trouts, and Chars). At sea, it is basically a silvery fish with spotting on the back, upper sides, top of head, and all the fins, including the tail fin. Spawning chinooks are olive brown to purplish or even red and undergo a radical metamorphosis, especially the males which develop a large kype. One way to distinguish the chinook from other species is by its black mouth and gums. The similar looking coho Oncorhynchus kisutch, has a black mouth, but white gums, except in the Great Lakes population where the gums may be gray or black.

It is the largest salmonid in North America, growing to at least 5 ft. (1.52 m) and 126 lb. (57 kg). It is an extremely important food and commercial fish, and due to its large size and game nature, an important sport fish. It is the only Pacific salmon in which the meat may be regularly either red or white, but the red meat commands a higher price.