Patrick Joseph Leahy OBE (/ˈleɪhi/ LAY-hee; 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.

Leahy was born in Montpelier, Vermont, the son of Alba (née Zambon) and Howard Francis Leahy. He has been legally blind in his left eye since birth. Leahy's maternal grandparents were Italian, and his father was of Irish ancestry; some of his ancestors came to Vermont in the 19th century to work at the granite quarries and manufacturing plants in Barre Town and Barre City. The Leahys ran a printing business across from the Vermont State House, and were also the publishers of the Waterbury Record newspaper. Leahy attended the parochial schools of Montpelier, and graduated from Montpelier's St. Michael's High School in 1957.

In 1961, Leahy graduated from Saint Michael's College with a Bachelor of Arts degree in government. While attending college, Leahy was a member of the National Federation of Catholic College Students. He was also involved with the Knights of Columbus, the Saint Michael's College Glee Club, and the school's pre-law society and politics club. He was active with the Reserve Officers' Training Corps rifle team, and was a member of the varsity rifle team. He was also on the staff of The Shield, the Saint Michael's College yearbook and WSSE, the school's AM radio station. In 1964, Leahy received his Juris Doctor from the Georgetown University Law Center. While in law school, Leahy was active in the Phi Delta Phi legal honor society. In addition, Leahy participated in Georgetown Law's Legal Aid Society and Legal Argument Program. He was also a representative to the school's Student Bar Association.

At graduation, Leahy was offered an E. Barrett Prettyman Fellowship, which would have enabled him to earn a Master of Laws degree from Georgetown while receiving training in courtroom advocacy. Leahy was also interviewed by the United States Attorney General, Robert F. Kennedy as one of several Georgetown Law students being considered for recruitment into the United States Department of Justice. He declined the fellowship and did not pursue a position at the Department of Justice because he intended to return to Vermont, and because he did not plan to practice criminal law.

Leahy was admitted to the bar soon after his law school graduation and became an associate at the Burlington firm headed by Philip H. Hoff, who then was governor of Vermont. In January 1965, Leahy was appointed as an assistant to Lewis E. Springer Jr., the legislative draftsman for the Vermont General Assembly. While working for Hoff's firm, Leahy was also appointed as Burlington's assistant city attorney.

In May 1966, Hoff appointed Leahy State's Attorney of Chittenden County after the incumbent resigned. Leahy was elected to a full term in 1966 and reelected in 1970. His service as state's attorney was notable for his participation in the sting operation that caught Paul Lawrence, an undercover police officer for numerous departments in Vermont. Lawrence falsely claimed to have purchased illegal drugs from several people, resulting in numerous convictions based on his perjury.