Agate Fossil Beds National Monument, Paleontological national monument in Sioux County, Nebraska, United States.

Agate Fossil Beds National Monument protects a stretch of the Niobrara River valley in Sioux County, Nebraska, where cliffs expose Miocene sedimentary rock. Two bluffs, Carnegie Hill and University Hill, hold dense bone beds of extinct mammals such as Menoceras and Stenomylus. The surrounding mixed-grass prairie grows prairie sandreed, blue grama and little bluestem.

The site began as Agate Springs Ranch before the United States government authorized it as a national monument in 1965. The official transformation to protected status took place in 1997.

The museum displays more than 500 Plains Indian artifacts that ranch owner James Cook gathered through his exchanges with Native American leaders. These objects tell the story of the people who lived on this land before the ranch was established.

The monument is remote, with no lodging or food service; bring water and supplies. Two picnic areas are available. Short trails cross the prairie and climb to the fossil-bearing cliffs above the Niobrara River. Of the 12.4 km² originally designated, about 11 km² are federal land; the rest belongs to two private farms.

The Devil's corkscrews are fossilized spiral burrows dug by Palaeocastor, a land-dwelling beaver ancestor, and were first studied at the site in the early 1900s. Each burrow descends in a helix to a chamber where the animal lived.