John Randolph Thune (; born January 7, 1961) is an American politician serving as the senior United States Senator from South Dakota since 2005. A member of the Republican Party, he previously served as a U.S. Representative for South Dakota's at-large congressional district from 1997 to 2003. He became South Dakota's senior senator with the retirement of Tim Johnson in 2015. He is South Dakota's senior senator, having served as the GOP's Chief Deputy Whip in 2006, chairman of the Republican Policy Committee in June 2009, and Republican Conference Chairman in 2011, the third-ranking position in the Senate. He has worked in politics and civic organizations since completing his MBA graduate degree. He is also the current dean of South Dakota's congressional delegation.