The Yellowstone bison herd roams the Greater Yellowstone Ecosystem. The bison herd is probably the oldest and largest public bison herd in the United States, estimated in 2020 to comprise 4,800 bison. The bison are American bison of the Plains bison subspecies. Yellowstone National Park may be the only location in the United States where free-ranging bison were never extirpated, since they continued to exist in the wild and were not reintroduced.

The plains bison is one of two subspecies/ecotypes of the American bison, the other being the wood bison. A natural population of plains bison survives in Yellowstone National Park and multiple smaller reintroduced herds of bison in many places in the United States as well as southern portions of the Canadian Prairies.