Patrick Joseph Leahy OBE (born March 31, 1940) is an American politician and attorney who represented Vermont in the United States Senate from 1975 to 2023. A member of the Democratic Party, he also was the president pro tempore of the United States Senate from 2012 to 2015 and from 2021 to 2023.

Leahy was the third-longest-serving U.S. senator in history at 48 years, and was the longest-serving member of the United States Congress to serve solely as a senator. During his tenure he chaired the Senate Appropriations Committee, the Senate Judiciary Committee and the Senate Agriculture Committee. In March 2022, he became the most senior member of Congress. At the time of his retirement, Leahy was the dean of Vermont's congressional delegation, Vermont's longest-serving U.S. senator, and the first Democrat ever elected to the U.S. Senate from Vermont. Leahy was succeeded by Peter Welch, who became the second Democrat to represent Vermont in the Senate.

Leahy's signature legislation is the Leahy Law forbidding the United States from funding foreign military units which violate human rights. Leahy was the presiding officer at Donald Trump's second impeachment trial, becoming the first senator to preside over a former president's impeachment trial. In 2023, Patrick Leahy Burlington International Airport was renamed after him.