Ronald Reagan's tenure as the 40th president of the United States began with his first inauguration on January 20, 1981, and ended on January 20, 1989. Reagan, a Republican from California, took office following his landslide victory over Democratic incumbent president Jimmy Carter and independent congressman John B. Anderson in the 1980 presidential election. Four years later in the 1984 presidential election, he defeated Democratic former vice president Walter Mondale to win re-election in a larger landslide. Reagan served two terms and was succeeded by his vice president, George H. W. Bush, who won the 1988 presidential election. Reagan's 1980 landslide election resulted from a dramatic conservative shift to the right in American politics, including a loss of confidence in liberal, New Deal, and Great Society programs and priorities that had dominated the national agenda since the 1930s.

Domestically, the Reagan administration enacted a major tax cut, sought to cut non-military spending, and eliminated federal regulations. The administration's economic policies, known as "Reaganomics", were inspired by supply-side economics. The combination of tax cuts and an increase in defense spending led to budget deficits, and the federal debt increased significantly during Reagan's tenure. Reagan signed the Tax Reform Act of 1986, simplifying the tax code by reducing rates and removing several tax breaks, and the Immigration Reform and Control Act of 1986, which enacted sweeping changes to U.S. immigration law and granted amnesty to three million illegal immigrants. Reagan also appointed more federal judges than any other president, including four Supreme Court Justices.

Reagan's foreign policy stance was resolutely anti-communist. Its plan of action, known as the Reagan Doctrine, sought to roll back the global influence of the Soviet Union in an attempt to end the Cold War. Under his doctrine, the Reagan administration initiated a massive buildup of the United States military; promoted new technologies such as missile defense systems; and in 1983 undertook an invasion of Grenada, the first major overseas action by U.S. troops since the end of the Vietnam War. The administration also created controversy by granting aid to paramilitary forces seeking to overthrow leftist governments, particularly in war-torn Central America and Afghanistan. Specifically, the Reagan administration engaged in covert arms sales to Iran to fund Contra rebels in Nicaragua that were fighting to overthrow their nation's socialist government. The resulting Iran–Contra affair led to the conviction or resignation of several administration officials. During Reagan's second term, he worked with Soviet leader Mikhail Gorbachev to sign a major arms control agreement. In 1986, Congress overrode Reagan's veto of a bill intended to implement economic sanctions against South Africa's apartheid regime.

Historians and political scientists generally rank Reagan in the upper tier of American presidents, and consider him to be one of the most important presidents since Franklin D. Roosevelt. Supporters of Reagan's presidency have pointed to his contributions to the economic recovery of the 1980s, the peaceful end of the Cold War, and a broader restoration of American confidence. Reagan's presidency has also received criticism for rising budget deficits and wealth inequality during and after his presidency. Due to Reagan's popularity with the public and advocacy of American conservatism, some historians have described the period during and after his presidency as the Reagan era.

Even prior to becoming president, Reagan was the leader of a dramatic conservative shift that undercut many of the domestic and foreign policies that had dominated the national agenda for decades. A major factor in the rise of conservatism was the growing distrust of government in the aftermath of the Watergate scandal. While distrust of high officials had been an American characteristic for two centuries, Watergate engendered heightened levels of suspicion and encouraged the media to engage in a vigorous search for scandals. An unexpected new factor was the emergence of the religious right as a cohesive political force that gave strong support to conservatism.

Other factors in the rise of the conservative movement were the emergence of a "culture war" as a triangular battle among conservatives, traditional liberals, and the New Left, involving such issues as individual freedom, divorce, sexual freedom, abortion, and homosexuality. A mass movement of population from the cities to the suburbs led to the creation of a new group of voters less attached to New Deal economic policies and machine politics. Meanwhile, it became socially acceptable for conservative Southern whites, especially well-educated suburbanites, to vote Republican. Though the civil rights legislation of the 1960s had been a triumphal issue for liberalism and had created a new, pro-Democratic black electorate, it had also destroyed the argument that whites had to vote Democratic to protect segregation in the South. Responding to these various trends, Reagan and other conservatives successfully presented conservative ideas as an alternative to a public that had grown disillusioned with New Deal liberalism and the Democratic Party. Reagan's charisma and speaking skills helped him frame conservatism as an optimistic, forward-looking vision for the country.

Reagan, who had served as Governor of California from 1967 to 1975, narrowly lost the 1976 Republican presidential primaries to incumbent president Gerald Ford. With the defeat of Ford by Democrat Jimmy Carter in the 1976 election, Reagan immediately became the front-runner for the 1980 Republican presidential nomination. A darling of the conservative movement, Reagan faced more moderate Republicans such as George H. W. Bush, Howard Baker, and Bob Dole in the 1980 Republican presidential primaries. After Bush won the Iowa caucuses, he became Reagan's primary challenger, but Reagan won the New Hampshire primary and most of the following primaries, gaining an insurmountable delegate lead by the end of March 1980. Ford was Reagan's first choice for his running mate, but Reagan backed away from the idea out of the fear of a "co-presidency" in which Ford would exercise an unusual degree of power. Reagan instead chose Bush, and the Reagan-Bush ticket was nominated at the 1980 Republican National Convention. Meanwhile, Carter won the Democratic nomination, defeating a primary challenge by Senator Ted Kennedy. Polls taken after the party conventions showed a tied race between Reagan and Carter, while independent candidate John B. Anderson had the support of many moderates.

The 1980 general campaign between Reagan and Carter was conducted amid a multitude of domestic concerns and the ongoing Iran hostage crisis. After winning the Republican nomination, Reagan pivoted to the center. Though he continued to champion a major tax cut, Reagan backed off from his support for free trade and the privatization of Social Security, and promised to consider arms control treaties with the Soviet Union. He instead sought to focus the race on Carter's handling of the economy. Mired with an approval rating in the low 30s, Carter also waged a negative campaign, focusing on the supposed risk of war if Reagan took office.