Sean Patrick Duffy (born October 3, 1971) is an American politician, attorney, and former television presenter serving as the 20th U.S. secretary of transportation since January 2025. Duffy has also served as the acting administrator of NASA since July 2025. A member of the Republican Party, he previously served as the U.S. representative for Wisconsin's 7th congressional district from 2011 to 2019 and as District Attorney of Ashland County from 2002 to 2010.

Born in Hayward, Wisconsin, Duffy was a professional lumberjack, competing in the city's Lumberjack World Championship for several years and winning multiple titles in speed climbing. In 1997, he appeared on The Real World: Boston. As a lumberjack, he competed in ESPN's Great Outdoor Games and was a commentator for the games. Duffy graduated from St. Mary's College of Minnesota with a degree in marketing and from the William Mitchell College of Law in 1999. He worked at his father's law practice for two years before serving as a special prosecutor for Ashland County. In 2002, Duffy was appointed by Wisconsin governor Scott McCallum as the district attorney of Ashland County, serving in the position until 2010.

In July 2009, Duffy announced that he would challenge Democratic U.S. representative Dave Obey. He defeated state senator Julie Lassa, a Democrat who won the nomination after Obey announced he would not run for re-election, in the 2010 election for Wisconsin's seventh congressional district. Duffy was re-elected in 2012, 2014, 2016, and 2018. Duffy resigned from Congress in September 2019 following his newborn daughter's health complications. After leaving Congress, Duffy worked for the consulting firm BGR Group. He began co-hosting Fox Business's The Bottom Line with Dagen McDowell in 2023.

On November 18, 2024, President-elect Donald Trump announced his intent to nominate Duffy for U.S. secretary of transportation. Duffy was confirmed by the U.S. Senate on January 28, 2025. In July 2025, Duffy was named as the acting administrator of NASA, succeeding Janet Petro.

Sean Patrick Duffy was born on October 3, 1971, in Hayward, Wisconsin. Duffy was the tenth child of Tom and Carol Duffy. He was raised in an Irish American Catholic family, several members of which participated in and won Hayward's annual Lumberjack World Championships. Duffy, inspired by his siblings, began logrolling when he was five. He attended Hayward High School, where he played as a defenseman for the school's hockey team. He began speed climbing when he was fifteen. Duffy continued lumberjacking after high school, winning a tree climbing title at the 1993 Mississippi River Log Boom. His speed climbing technique involved reckless movement; Fred Scheer compared Duffy to a "race-car driver who bumps the wall now and then". Duffy was the world champion speed climber at the 1994 Lumberjack World Championship.

He graduated from Saint Mary's University of Minnesota with a B.A degree in marketing in 1994 and worked for Scheer's Lumberjack Shows in the summers. By June 1997, The New York Times Magazine considered Duffy one of the top American lumberjacks at that time. He later graduated from the William Mitchell College of Law with a J.D. degree in 1999. Duffy's nephew is Erik Johnson, the longtime Colorado Avalanche defenseman who was the first overall pick in the 2006 NHL entry draft.

In 1997, Duffy auditioned for The Real World: Boston, the sixth iteration of MTV's The Real World set in Boston, Massachusetts, and became a cast member by July. In his audition, Duffy said that he was interested in "cute girls"; his role on the show was described by The New York Times as a "resident playboy". The series completed filming in December, though Duffy would begin filming for Road Rules: All Stars (1998) that month, necessitating him to delay another semester of law school. On the set of Road Rules, he met Rachel Campos. The two married in April 1999 at the Newman Center at Arizona State University located within Old St. Mary's Church; though they planned to wed later that year, Campos's unexpected pregnancy expedited their wedding. The two have nine children together. In 2002, Duffy appeared in and won Real World/Road Rules Challenge: Battle of the Seasons. He competed in the Great Outdoor Games's competitions for log rolling in 2000 and 2001 and for speed climbing from 2000 to 2002. In 2003, Duffy was named as a commentator for the Great Outdoor Games. He was named as the honorary athlete for the 2004 Badger State Winter Games. Duffy competed in the speed climbing competition for the Great Outdoor Games in 2005. Additionally, he was a commentator for the mixed doubles Boom Run.

After graduating from law school in 1999, Duffy worked for his father's practice for two years before becoming a special prosecutor for Ashland County. In June 2002, Wisconsin governor Scott McCallum appointed Duffy as the district attorney of Ashland County following Michael Gableman's resignation. Duffy told The Post-Crescent that he intended to run for a complete term in that year's election. He ran unopposed in that year's election and the 2004 election. Duffy served as a Wisconsin delegate in the 2004 Republican National Convention and the 2004 presidential election. In May 2005, Wisconsin's Federal Nominating Commission forwarded four candidates, including Duffy, to succeed J. B. Van Hollen as the United States attorney for the Western District of Wisconsin. Duffy was re-elected in 2006 and 2008. As district attorney, he had a ninety percent conviction rate.