Patrick Joseph Leahy (born March 31, 1940) is the senior United States Senator from Vermont. He is a member of the Democratic Party, having the distinction of being the first and only Democratic United States Senator in Vermont's history, and is the current chairman of the Senate Judiciary Committee. As of 2009 Leahy is also fourth most senior U.S. Senator.

Leahy was born in Montpelier, Vermont, the son of Alba (née Zambon) and Howard Francis Leahy, a printer. His father was Irish-American and his mother Italian-American. His family roots are traced back to Irish immigrants in Montreal, Quebec and his wife's family is from Quebec. He has a brother, John, and a sister, Mary.

Leahy graduated from Saint Michael's College in 1961 and received his J.D. degree from Georgetown University Law School in 1964. He practiced as a lawyer until he was elected for four terms as State's Attorney of Chittenden County from 1966 to 1974. Leahy was elected to the United States Senate for the first time in 1974 (at 34, he was the youngest U.S. Senator ever to be elected by Vermont), and has been re-elected five times. Leahy was the first Democrat elected to Congress from Vermont since the Civil War, and remains the only Democrat to have been elected Senator from Vermont, although Bernie Sanders, I-Vermont, received the endorsement of the Vermont Democratic Party when he ran as an independent in 2006.