violent confrontation between police and labour protesters in Chicago on May 4, 1886, that dramatized the labour movement's struggle for recognition in the United States.

On May 3, one person was killed and several were injured as police intervened to protect strikebreakers from strikers during a union action at the McCormick Harvesting Machine Company that was part of a larger campaign to secure an eight-hour workday (labour, hours of). To protest police brutality, radicals held a mass meeting the next day in Haymarket Square. The meeting remained peaceful until police attempted to disperse it, whereupon a bomb was thrown by an individual never positively identified. Seven policemen were killed and 60 others were injured before the violence ended.

Amid the panic (collective behaviour) that followed the resulting riot, August Spies and seven other alleged anarchist (anarchism) labour leaders were convicted of murder on the grounds that they had conspired with or aided an unknown assailant. Many of the eight arrested, however, were not even present at the May 4 event, and their alleged involvement was never proved. Nevertheless, Spies and three other defendants were hanged on November 11, 1887, while another committed suicide. The surviving three were pardoned in 1893 by Illinois Governor John Peter Altgeld (Altgeld, John Peter)—an action widely condemned by industrialists but applauded by labour reformers.

The Haymarket Riot had a lasting effect on the labour movement in the United States. The Knights of Labor (KOL), at the time the largest and most successful union organization in America, was blamed for the riot. While the KOL also had sought an eight-hour day and had called several strikes (strike) to achieve the goal, its involvement in the riot could not be proved. Public distrust, however, caused many KOL locals to join the newly formed and less-radical American Federation of Labor (American Federation of Labor–Congress of Industrial Organizations).