John Randolph Thune (/ˈθuːn/ THOON; born January 7, 1961) is an American politician serving as the senior United States senator from South Dakota, a seat he has held since 2005. A Republican, Thune has been the Senate majority leader and Senate Republican leader since January 2025. He is in his fourth Senate term.

A South Dakota native, Thune is a graduate of Biola University (BA) and the University of South Dakota (MBA). From 1997 to 2003, he served three terms as the U.S. representative for South Dakota's at-large congressional district. He first ran for the U.S. Senate in 2002, narrowly losing to incumbent senator Tim Johnson. In 2004, Thune ran for Senate again and defeated Senate Democratic leader Tom Daschle, making Daschle the first incumbent Senate leader to lose an election since 1952. During his Senate tenure, Thune has served as the Republican chief deputy whip (2007-2009); chair of the Senate Republican Policy Committee (2009–2012); Senate Republican Conference chair, the third-ranking position in the Senate (2012–2019); majority whip (2019–2021); and minority whip (2021–2025).

In 2024, Thune was elected Senate Republican leader, succeeding Mitch McConnell.

Thune was born in Pierre, South Dakota, on January 7, 1961. He is the son of Yvonne Patricia (née Bodine) and Harold Richard Thune. Harold Thune was a fighter pilot in the Pacific theater during World War II who flew the Grumman F6F Hellcat; he was awarded the Distinguished Flying Cross after shooting down four enemy planes. Harold Thune flew his missions off the USS Intrepid. Thune's paternal grandfather, Nicholas Thune, emigrated to the United States from Norway in 1906; he partnered with his brother to run a chain of hardware stores in South Dakota. Nicholas Thune changed his last name to Thune from Gjelsvik because he was told by immigration officers that the name Gjelsvik was too hard to pronounce. Thune's maternal grandfather was from Ontario, Canada, and his mother was born in Saskatchewan.

Thune was a star athlete in high school, active in basketball, track, and football. He graduated from Jones County High School in 1979. Thune played college basketball at Biola University in California; he graduated from Biola in 1983 with a Bachelor of Arts degree in business. He received a Master of Business Administration degree from the University of South Dakota in 1984.

After completing his MBA, Thune became involved in politics. He worked as a legislative aide for U.S. senator James Abdnor from 1985 to 1987. In 1989, Thune moved to Pierre, where he served as executive director of the state Republican Party for two years. Thune was appointed Railroad Director of South Dakota by Governor George S. Mickelson and served from 1991 to 1993. From 1993 to 1996, he was executive director of the South Dakota Municipal League.

Thune began his political career in 1996 by entering the race for South Dakota's lone seat in the U.S. House of Representatives. The Almanac of American Politics said that Thune "entered the 1996 race as very much an underdog." His opponent in the Republican primary was sitting Lieutenant Governor Carole Hillard of Rapid City, who benefited from the support of the longtime South Dakota Governor Bill Janklow. A May 1996 poll showed Hillard leading Thune by a margin of 69%–15%. By relying on strong personal skills and the help of his old network of Abdnor friends, Thune won the primary, defeating Hillard 59%–41%. In the general election, Thune defeated Democrat Rick Weiland, a long-serving aide to U.S. Senator Tom Daschle, 58%–37%.

Thune won his subsequent U.S. House races by wide margins. He was reelected in 1998 with 75% of the vote and in 2000 with 73% of the vote.