The Haymarket affair (also known as the Haymarket massacre, the Haymarket riot, the Haymarket Square riot, or the Haymarket Incident) was the aftermath of a bombing that took place at a labor demonstration on May 4, 1886, at Haymarket Square in Chicago, Illinois, United States. It began as a peaceful rally in support of workers striking for an eight-hour work day, the day after police killed one and injured several workers. An unknown person threw a dynamite bomb at the police as they acted to disperse the meeting, and the bomb blast and ensuing gunfire resulted in the deaths of seven police officers and at least four civilians; dozens of others were wounded.

In the internationally publicized legal proceedings that followed, eight anarchists were convicted of conspiracy. The evidence was that one of the defendants may have built the bomb, but none of those on trial had thrown it. Seven were sentenced to death and one to a term of 15 years in prison. Illinois Governor Richard J. Oglesby commuted two of the sentences to terms of life in prison; another committed suicide in jail rather than face the gallows. The other four were hanged on November 11, 1887. In 1893, Illinois Governor John Peter Altgeld pardoned the remaining defendants and criticized the trial.The Haymarket Affair is generally considered significant as the origin of International Workers' Day held on May 1, and it was also the climax of the social unrest among the working class in America known as the Great Upheaval. According to labor historian William J. Adelman:

No single event has influenced the history of labor in Illinois, the United States, and even the world, more than the Chicago Haymarket Affair. It began with a rally on May 4, 1886, but the consequences are still being felt today. Although the rally is included in American history textbooks, very few present the event accurately or point out its significance.

The site of the incident was designated a Chicago landmark in 1992, and a sculpture was dedicated there in 2004. In addition, the Haymarket Martyrs' Monument was designated a National Historic Landmark in 1997 at the defendants' burial site in Forest Park.