Yellowstone National Park (Arapaho: Henihco'oo' or Héetíhco'oo[4]) is a national park located primarily in the U.S. state of Wyoming, although it also extends into Montana and Idaho. It was established by the U.S. Congress and signed into law by President Ulysses S. Grant on March 1, 1872. Yellowstone, widely held to be the first national park in the world, is known for its wildlife and its many geothermal features, especially Old Faithful Geyser, one of the most popular features in the park. It has many types of ecosystems, but the subalpine forest is dominant.

Native Americans have lived in the Yellowstone region for at least 11,000 years. The region was bypassed during the Lewis and Clark Expedition in the early 19th century. Aside from visits by mountain men during the early-to-mid-19th century, organized exploration did not begin until the late 1860s. The U.S. Army was commissioned to oversee the park just after its establishment. In 1917, administration of the park was transferred to the National Park Service, which had been created the previous year. Hundreds of structures have been built and are protected for their architectural and historical significance, and researchers have examined more than 1,000 archaeological sites.

Yellowstone National Park spans an area of 3,468.4 square miles (8,983 km2), comprising lakes, canyons, rivers and mountain ranges. Yellowstone Lake is one of the largest high-altitude lakes in North America and is centered over the Yellowstone Caldera, the largest supervolcano on the continent. The caldera is considered an active volcano. It has erupted with tremendous force several times in the last two million years. Half of the world's geothermal features are in Yellowstone, fueled by this ongoing volcanism. Lava flows and rocks from volcanic eruptions cover most of the land area of Yellowstone. The park is the centerpiece of the Greater Yellowstone Ecosystem, the largest remaining, nearly intact ecosystem in the Earth's northern temperate zone.

Hundreds of species of mammals, birds, fish and reptiles have been documented, including several that are either endangered or threatened. The vast forests and grasslands also include unique species of plants. Yellowstone Park is the largest and most famous megafauna location in the Continental United States. Grizzly bears, wolves, and free-ranging herds of bison and elk live in the park. The Yellowstone Park bison herd is the oldest and largest public bison herd in the United States. Forest fires occur in the park each year; in the large forest fires of 1988, nearly one third of the park was burnt. Yellowstone has numerous recreational opportunities, including hiking, camping, boating, fishing and sightseeing. Paved roads provide close access to the major geothermal areas as well as some of the lakes and waterfalls. During the winter, visitors often access the park by way of guided tours that use either snow coaches or snowmobile.