Todd existed as a former county from 1862 to 1899. Most was transferred to Nebraska in 1882, mainly to what was reported as Boyd County in 1890; remainder annexed to Gregory in 1899.
Todd was re-instantiated as a current county in 1909.
Todd County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 9,612. Todd County does not have its own county seat. Instead, Winner in neighboring Tripp County serves as its administrative center. Its largest city is Mission. The county was named by European-American settlers after John Blair Smith Todd, who was a delegate from Dakota Territory to the United States House of Representatives and a general in the Union Army during the American Civil War.
The county lies entirely within the Rosebud Indian Reservation and is coterminous with the main reservation (exclusive of off-reservation trust lands, which lie in four nearby counties). Its southern border is with the state of Nebraska. It is one of five South Dakota counties entirely within an Indian reservation. (The others are Corson, Dewey, Shannon, and Ziebach.) The county's per-capita income makes it the third poorest county in the United States. Unlike many rural counties in South Dakota, since 1960, its population has increased.