Ku Klux Klan (KKK) is the name of several past and present organizations in the United States that advocated white supremacy, antisemitism, anti-Catholicism, racism, homophobia, anti-Communism and nativism. These organizations used terrorism, violence, lynching and cross lighting to oppress African Americans and other religious, social or ethnic groups.

The first Klan was founded in 1866 by veterans of the Confederate Army. Its purpose was to resist Reconstruction by intimidating "carpetbaggers," "scalawags" and freed slaves. The KKK quickly adopted violent methods, causing a backlash by many Southern elites who saw the Klan as an excuse for federal troops to continue their activities. The organization declined from 1868 to 1870 and was destroyed by President Ulysses S. Grant's action under the Civil Rights Act of 1871.

In 1915, the second Klan was founded using the same name. The film The Birth of a Nation and the sensationalized newspaper coverage of the trial, conviction and lynching of Leo Frank of Georgia sparked the Klan's revival. It grew amid social fears aroused by rapid changes in many major cities absorbing immigrants from southern and eastern Europe; and the Great Migration of Southern blacks and whites from rural areas. The second KKK was a formal fraternal organization, with a national and state structure, that paid thousands of men to organize local chapters all over the country. At its peak in the mid-1920s, the organization included about 15% of the nation's eligible population, approximately 4–5 million men. The second KKK typically preached racism, anti-Catholicism, anti-Communism, nativism, and anti-Semitism. Some local groups took part in lynchings and other violent activities. Violence occurred mostly in the South, which already had a tradition of lawlessness. Its popularity fell during the Great Depression, and membership fell further during World War II because of scandals resulting from prominent members' crimes and its support of Nazi Germany.

The name "Ku Klux Klan" has since been used by many independent groups. Many of them opposed the Civil Rights Movement and desegregation in the 1950s and 1960s. During that period, they often acted with impunity by alliances with Southern police departments, as during the reign of Bull Connor in Birmingham, Alabama; or even governor's offices, as with George Wallace of Alabama. Several members of KKK-affiliated groups were eventually convicted of murder and manslaughter in the deaths of civil rights workers and children Alabama and Mississippi (as in the bombing of the 16th Street Baptist Church in Alabama, the assassination of NAACP organizer Medgar Evers, and the murders of three civil rights workers). Today, researchers estimate that there are as many as 150 Klan chapters with up to 8,000 members nationwide. The US government classifies these groups, with operations in separated small local units, as extreme hate groups. The modern KKK has been repudiated by all mainstream media, political and religious leaders.