The Yellowstone expedition was an expedition to the American frontier in 1819 and 1820 authorized by United States Secretary of War John C. Calhoun, with the goal of establishing a military fort or outpost at the mouth of the Yellowstone River in present-day North Dakota. Sometimes called the Atkinson–Long Expedition after its two principal leaders, Colonel Henry Atkinson and Major Stephen Harriman Long, it led to the creation of Fort Atkinson in present-day Nebraska, the first United States Army post established west of the Missouri River, but was otherwise a costly failure, stalling near Council Bluffs, Iowa.

Henry Atkinson was a United States army officer serving on the western frontier during the War of 1812 and the Yellowstone expedition. With Benjamin O'Fallon, he negotiated treaties with Indigenous nations of the upper Missouri River in 1825. Over his career in the army, he served in the West, the Gulf Coast, and in New York at the border with The Canadas.