Marion Michael Rounds (born October 24, 1954) is an American businessman and politician serving as the junior United States Senator from South Dakota since 2015. A member of the Republican Party, he served as the 31st Governor of South Dakota from 2003 to 2011, having previously served in the South Dakota Senate from 1991 to 2001.

He holds the U.S. Senate seat previously held by retired Democrat Tim Johnson.