McCarthyism, also known as the second Red Scare, was the political repression and persecution of left-wing individuals and a campaign spreading fear of alleged communist and socialist influence on American institutions and of Soviet espionage in the United States during the late 1940s through the 1950s. After the mid-1950s, U.S. Senator Joseph McCarthy, who had spearheaded the campaign, gradually lost his public popularity and credibility after several of his accusations were found to be false. The U.S. Supreme Court under Chief Justice Earl Warren made a series of rulings on civil and political rights that overturned several key laws and legislative directives, and helped bring an end to the Second Red Scare. Historians have suggested since the 1980s that as McCarthy's involvement was less central than that of others, a different and more accurate term should be used instead that more accurately conveys the breadth of the phenomenon, and that the term McCarthyism is now outdated. Ellen Schrecker has suggested that Hooverism after FBI Head J. Edgar Hoover is more appropriate.

What would become known as the McCarthy era began before McCarthy's rise to national fame. Following the breakdown of the wartime East-West alliance with the Soviet Union, and with many remembering the First Red Scare, President Harry S. Truman signed an executive order in 1947 to screen federal employees for possible association with organizations deemed "totalitarian, fascist, communist, or subversive", or advocating "to alter the form of Government of the United States by unconstitutional means." The following year, the Czechoslovak coup by the Communist Party of Czechoslovakia heightened concern in the West about Communist parties seizing power and the possibility of subversion. In 1949, a high-level State Department official was convicted of perjury in a case of espionage, and the Soviet Union tested a nuclear bomb. The Korean War started the next year, significantly raising tensions and fears of impending communist upheavals in the United States. In a speech in February 1950, McCarthy claimed to have a list of members of the Communist Party USA working in the State Department, which attracted substantial press attention, and the term McCarthyism was published for the first time in late March of that year in The Christian Science Monitor, along with a political cartoon by Herblock in The Washington Post. The term has since taken on a broader meaning, describing the excesses of similar efforts to crack down on alleged "subversive" elements. In the early 21st century, the term is used more generally to describe reckless and unsubstantiated accusations of treason and far-left extremism, along with demagogic personal attacks on the character and patriotism of political adversaries.

The primary targets for persecution were government employees, prominent figures in the entertainment industry, academics, left-wing politicians, and labor union activists. Suspicions were often given credence despite inconclusive and questionable evidence, and the level of threat posed by a person's real or supposed leftist associations and beliefs were often exaggerated. Many people suffered loss of employment and the destruction of their careers and livelihoods as a result of the crackdowns on suspected communists, and some were outright imprisoned. Most of these reprisals were initiated by trial verdicts that were later overturned, laws that were later struck down as unconstitutional, dismissals for reasons later declared illegal or actionable, and extra-judiciary procedures, such as informal blacklists by employers and public institutions, that would come into general disrepute, though by then many lives had been ruined. The most notable examples of McCarthyism include the investigations of alleged communists that were conducted by Senator McCarthy, and the hearings conducted by the House Un-American Activities Committee (HUAC).