John Randolph Thune is an American politician serving as the senior United States senator from South Dakota, since 2005. A member of the Republican Party, he has been the Senate majority leader and Senate Republican leader since 2025. Thune is in his fourth Senate term and is the dean of South Dakota's congressional delegation. From 1997 to 2003, he served three terms as the U.S. representative for South Dakota's at-large congressional district.