The Haymarket riot, also known as the Haymarket massacre, the Haymarket Affair, or the Haymarket Square riot is a seminal event in labor history. It came at a time of growth in the U.S. economy as well as economic uncertainty and changes in the industrial sector. The years after the Civil War saw a growth in the labor movement and the rise of groups like the Federation of Organized Trades and Labor Unions, and Knights of Labor. It also saw the rise of anarchist groups.

Chicago was a rising industrial city at the center of the economic changes. It was also a focus for the growing labor movement that was pushing for better working conditions including an 8-hour workday. On May 1, 1886, labor groups called for a general strike with an emphasis on an 8-hour workday. There were strikes in a number of cities including one in Chicago where anarchist Albert Parsons along with his wife Lucy, led a procession down Michigan Avenue. Two days later August Spies spoke at a rally outside of the McCormick reaper plant where at least one person was killed after attendees surged forward and were fired on by police.

In response to those events, local anarchists printed fliers for a rally the next day at Haymarket Square. The rally began peacefully, and featured speakers included August Spies, Albert Parsons, and the Rev. Samuel Fielden. As the weather turned bad, many attendees had already started leaving, but police began to further disperse the crowd. Someone threw a home-made bomb and there was an exchange of gunfire. In the end, seven police officers and several civilians were killed, while dozens of others were wounded.

The offices of Arbeiter-Zeitung, a German-language radical newspaper, were raided and its editor August Spies along with employees Michael Schwab and Adolph Fischer were arrested. Others were also questioned or arrested, and the trial for those arrested was scheduled for the next year. The eight defendants that went on trial on June 21 were Albert Parsons, Adolph Fisher, August Spies, George Engle, Louis Lingg, Michael Schwab, Samuel Fielder, and Oscar Neebee—all were found guilty. All but Neebee were sentenced to death. Lingg died by suicide while in prison, while Parsons, Fisher, Spies, and Engle were executed on November 11, 1887. In the wake of the events there was an anti-union clampdown, and many immigrants came under suspicion.

The events at Haymarket Square have left their impact on labor history and a part of American history and have left their mark. In June 1883, a Haymarket Martyrs' Monument was dedicated at Forest Home Cemetery in Forest Park, Illinois and in 1997, it was designated a National Historic Landmark. Additionally, Haymarket has generally been considered important to the creation of International Workers' Day held on May 1.