Pompeys Pillar National Monument, Natural sandstone monument in Yellowstone County, Montana, United States

Pompeys Pillar is a sandstone formation rising roughly 150 feet above the Yellowstone River along a protected riverside area in south central Montana. The site features thousands of carved marks across its surface and includes a visitor center with trails and educational displays about the region.

The sandstone butte served as a landmark for about 11,000 years before William Clark, separated from Meriwether Lewis on the return journey, explored the Yellowstone River as the first white man and carved his name and the date July 25, 1806 into the rock. He named it after Jean Baptiste Charbonneau, Sacagawea's son, nicknamed Pompey; the current name dates from 1814. It became a National Historic Landmark on July 25, 1965, and a National Monument on January 17, 2001.

The Crow name for the rock, Iishbiiammaache, means "Place of the Mountain Lion." Cheyenne, Lakota, Shoshone, Arapaho, Blackfeet and Salish peoples also have names for it. Petroglyphs mark it as a resting place used by many visitors, and the site remains a landmark for travellers along the Yellowstone River.

The rock is easily reached via marked trails that branch from the visitor center. The best time to visit is from May through September, when the center operates and trails are accessible.

The monument records a timeline of roughly eleven thousand years of human activity, as traces from Native Americans, traders, and later visitors layer across the entire surface. This layering creates a natural archive of visitor marks spanning many millennia.