Gary Warren Hart (born November 28, 1936) is an American politician, diplomat, and lawyer. He was the front-runner for the 1988 Democratic presidential nomination until he dropped out amid revelations of extramarital affairs. He represented Colorado in the United States Senate from 1975 to 1987.

Born in Ottawa, Kansas, Hart pursued a legal career in Denver, Colorado, after graduating from Yale Law School. He managed Senator George McGovern's successful campaign for the 1972 Democratic presidential nomination and McGovern's unsuccessful general election campaign against President Richard Nixon. Hart defeated incumbent Republican Senator Peter Dominick in Colorado's 1974 Senate election. In the Senate, he served on the Church Committee and led the Senate investigation regarding the Three Mile Island accident. After narrowly winning re-election in 1980, he sponsored the Semiconductor Chip Protection Act of 1984, becoming known as an "Atari Democrat".

Hart sought the Democratic presidential nomination in 1984, narrowly losing the race to former Vice President Walter Mondale. Hart declined to seek re-election to the Senate in 1986 and sought the Democratic presidential nomination in 1988. He was widely viewed as the front-runner until reports surfaced of an extramarital affair, and Hart withdrew from the race in May 1987. He re-entered the race in December 1987 but withdrew from the race again after faring poorly in the early primaries. The nomination ultimately went to Michael Dukakis.

Hart returned to private practice after the 1988 election and served in a variety of public roles. He co-chaired the Hart-Rudman Task Force on Homeland Security, served on the Homeland Security Advisory Council, and was the United States Special Envoy for Northern Ireland. He earned a doctorate in politics from the University of Oxford and has written for outlets such as The Huffington Post. He has also written several books, including a biography of President James Monroe. Hart married Lee Ludwig in 1958, who died at age 85 on April 9, 2021. They had two children, John and Andrea Hart.