Charles Sumner Eastman (January 23, 1864 – August 26, 1939) was an American politician and lawyer.
Born in Primrose, Wisconsin, Eastman was educated in the Dane County public schools and at the Northwestern Business College. In 1882, he moved to Dakota Territory and settled in Plankinton, Dakota Territory. In 1888, he was admitted to the Dakota Territory bar and practiced law in what is now Hot Springs, South Dakota and was senior member of Eastman & Dudley Law Firm. He served as county judge of Fall River County, South Dakota 1887-1889 and then was deputy sheriff and sheriff of Fall River County 1892-1897. He served as a Democrat in the South Dakota House of Representatives. His uncle was United States Robert M. La Follette, Sr. of Wisconsin. Eastman died in Hot Springs, South Dakota.