The Yankton Treaty was a treaty signed in 1858 between the United States government and the Yankton Sioux (Nakota) Native American tribe, ceding most of eastern South Dakota to the United States government. The treaty was signed in April 1858, and ratified by the United States Congress on February 17, 1859. The agreement immediately opened this territory up for settlement by whites, resulting in the establishment of an unofficial local government not recognized by Washington. The treaty also created the 400,000 acre Yankton Sioux Reservation, located in present-day Charles Mix County in South Dakota.