Paul Efthemios Tsongas (/ˈsɒŋɡəs/ SONG-gəs; February 14, 1941 – January 18, 1997) was an American politician who represented Massachusetts in the United States Senate from 1979 until 1985 and in the United States House of Representatives from 1975 until 1979. A member of the Democratic Party, he ran for president in 1992. He won eight contests during the presidential primaries but ultimately lost the nomination to Bill Clinton, who later won the general election.

Born in Lowell, Massachusetts, Tsongas graduated from Dartmouth College, Yale Law School and the Kennedy School of Government. After working for the Peace Corps and as an aide to Congressman F. Bradford Morse, Tsongas successively won election as a city councilor and county commissioner. In 1974, Tsongas was elected to the House of Representatives representing Massachusetts's 5th congressional district, after defeating incumbent Paul W. Cronin. In 1978, he ran for the Senate, and defeated incumbent Republican Edward Brooke.

In Congress, Tsongas established a reputation as a social liberal and fiscal conservative. Tsongas was diagnosed with non-Hodgkin lymphoma in 1983 and declined to seek re-election in 1984. He returned to politics after undergoing a successful bone marrow transplant. He experienced early success in the 1992 Democratic presidential primaries, winning the New Hampshire primary, but withdrew from the race in March 1992 and endorsed Clinton. An opponent of deficit spending, Tsongas co-founded the Concord Coalition. He died in 1997 of complications from pneumonia and non-Hodgkin lymphoma.

Tsongas was born in Lowell, Massachusetts, along with a twin sister, Thaleia, to a once working-class family who came to own a very successful dry cleaning business in Lowell. His father, Efthemios George Tsongas, was a Greek immigrant, and his mother, Katina (née Pappas; originally Panagiotopoulos), was of Greek descent.

After graduating from Lowell High School in 1958, Tsongas attended Dartmouth College, graduating in 1962 with a B.A. in economics, then Yale Law School and the John F. Kennedy School of Government at Harvard University before settling in Lowell, Massachusetts. While at Dartmouth, Tsongas was a member of the men's swimming team. He picked up the sport again, 27 years later, in 1986, after doctors suggested swimming as a way to rebuild his lung capacity while he was recovering from lymphoma. At a YMCA Masters meet, Tsongas once remarked, "I always used to say breaststroke was an athletic event and the butterfly was the political statement."

Tsongas served as a Peace Corps volunteer in Ethiopia from 1962 to 1964, and as Peace Corps Country Director in the West Indies from 1967 to 1968. In 1967, Tsongas – working as an aide to Congressman F. Bradford Morse – met Niki Sauvage, who was spending the summer in Arlington, Virginia. They were married in 1969, and had three daughters: Ashley, Katina, and Molly. Niki Tsongas was a U.S. Representative from Massachusetts from 2007 to 2019. When Tsongas ran for office, out of concern that people would pronounce the silent T in his last name, the campaign distributed bumper stickers saying "Tsenator Tsongas".

Tsongas first entered politics as a city councilor, elected to the Lowell City Council in 1969 where he served two consecutive terms. Tsongas went on to serve as a county commissioner of Middlesex County, Massachusetts. In 1974, he ran for United States House of Representatives from a district anchored by Lowell. The district had elected only three Democrats in its entire existence and had been in Republican hands continuously since 1895. However, in the massive Democratic wave of the post-Watergate election of 1974, he defeated freshman Republican Paul W. Cronin by a 21-point margin. He was reelected in 1976, becoming the first Democrat to hold the district for more than one term.

Increasingly popular and well-liked in Massachusetts, in 1978 he was elected to the United States Senate, defeating incumbent Republican Edward Brooke by 202,699 votes.