Thomas Richard Harkin (born November 19, 1939) is an American lawyer, author, and politician who served as a United States senator from Iowa from 1985 to 2015. A member of the Democratic Party, he previously was the U.S. representative for Iowa's 5th congressional district from 1975 to 1985. He is the longest-serving senator to spend the entire tenure as a state's junior senator.

Born in Cumming, Iowa, Harkin graduated from Iowa State University and The Catholic University of America's Columbus School of Law. He served in the United States Navy as an active-duty jet pilot (1962–1967). After serving as a congressional aide for several years, he made two runs for the U.S. House of Representatives, losing in 1972 but winning in 1974. He went on to serve five terms in the House.

Harkin won a race for U.S. Senate in 1984 by a wide margin. He was an early frontrunner for his party's presidential nomination in 1992, but he dropped out in support of eventual winner Bill Clinton. He served five Senate terms and at the end of his time in the Senate served as chair of the Senate Committee on Health, Education, Labor, and Pensions. He authored the Americans with Disabilities Act of 1990 and was its chief sponsor in the Senate. Harkin delivered part of his introduction speech in sign language, saying it was so his deaf brother could understand.

On January 26, 2013, Harkin announced that he would not seek reelection in 2014.