Ronald Harold Johnson (born April 8, 1955) is an American businessman and far-right politician serving as the senior United States senator from Wisconsin, a seat he has held since 2011. A Republican, Johnson was first elected to the U.S. Senate in 2010, defeating Democratic incumbent Russ Feingold. He was reelected in 2016, defeating Feingold in a rematch, and in 2022, narrowly defeating Lieutenant Governor Mandela Barnes.

Born in Mankato, Minnesota, Johnson attended high school in Edina, Minnesota, a suburb of the Twin Cities, and received a degree from the University of Minnesota. Before entering politics, he was chief executive officer of a polyester and plastics manufacturer in Oshkosh, Wisconsin, founded by his brother-in-law.

A staunch ally of President Donald Trump, Johnson voted for Trump's Tax Cuts and Jobs Act in 2017, supported Trump's decision to end Deferred Action for Childhood Arrivals (DACA), launched investigations into his political opponents and promoted false claims of fraud in relation to Trump's defeat in the 2020 presidential election. He has rejected the scientific consensus on climate change. During the COVID-19 pandemic, Johnson voted for the CARES Act, resisted stay at home orders, used his chairmanship of the Senate Homeland Security Committee to invite witnesses who promoted fringe theories about COVID-19 and spread misinformation about COVID-19 vaccinations. He has also suggested Social Security and Medicare spending be subject to an annual congressional vote.