Our paper discusses how the Ku Klux Klan (Klan) is the most enduring form of vigilantism in United States history, extending from the 1870s through today. We show how the Klan fomented vigilantism against particular groups of people in four eras: the 1870s, immediately after the Civil War over slavery; the interwar 1920s, which was characterized by high rates of immigration from Europe; the 1950s–1960s, which witnessed legal and political challenges to racial segregation or racially exclusive voting practices in the southern states; and the 1980s–2000s, during which the Klan allied with other far-right racist groups to forge a Pan-Aryan alliance. We discuss each era of the Klan within its historical context to explain how it justified and gained popular support for vigilante agendas and practices, which we define as actions or serious threats of extra-legal violence that replace or enhance the legitimated violence of the state such as the police, courts, and military. We pay attention to the wider context in which the Klan developed, how it was organized, its principle vigilante activities and strategies, and its relationship to electoral and government actors.