The New Orleans massacre of 1866 occurred on July 30, when a peaceful demonstration of mostly Black Freedmen was set upon by a mob of white rioters, many of whom had been soldiers of the recently defeated Confederate States of America, leading to a full-scale massacre. The violence erupted outside the Mechanics Institute, site of a reconvened Louisiana Constitutional Convention. According to the official report, a total of 38 were killed and 146 wounded, of whom 34 dead and 119 wounded were Black Freedmen. Unofficial estimates were higher. Gilles Vandal estimated 40 to 50 Black Americans were killed and more than 150 Black Americans wounded. Others have claimed nearly 200 were killed. In addition, three white convention attendees were killed, as was one white protester.

During much of the American Civil War, New Orleans had been occupied and under martial law imposed by the Union. On May 12, 1866, Mayor John T. Monroe, a Democrat who had ardently supported the Confederacy, was reinstated as acting mayor, the position he held before the war. Judge R. K. Howell was elected as chairman of the convention, with the goal of increasing participation by voters likely to vote for removal of the Black Codes.

The massacre expressed conflicts deeply rooted in the social structure of Louisiana. The New Orleans massacre was a continuation of a longer shooting war over slavery (beginning with Bleeding Kansas in 1859), of which the 1861–1865 hostilities were merely the largest part. More than half of the whites were Confederate veterans and nearly half of the Black Americans were veterans of the Union army. The national reaction of outrage at the earlier Memphis riots of 1866 and the New Orleans massacre helped the Radical Republicans win a majority in both houses of Congress in the 1866 midterm elections. The riots catalyzed support for the Fourteenth Amendment, extending suffrage and full citizenship to freedmen, and the Reconstruction Act, to establish military districts for the national government to oversee areas of the South and work to change their social arrangements.

The State Constitutional Convention of 1864 authorized greater civil freedoms to Blacks within Louisiana yet provided no voting rights for any people of color. Free people of color, who were mixed-race, had been an important part of New Orleans for more than a century and were established as a separate class in the colonial period, before United States annexation of the territory in 1803. Many were educated and owned property and were seeking the vote. In addition, Republicans had the goals of extending the suffrage to freedmen and eliminating the Black Codes passed by the legislature. They reconvened the convention and succeeded in incorporating these goals.

White Democrats by and large considered the reconvened convention illegal, as they said that the voters, then limited to whites only, had accepted the constitution. White supremacy was a plank of their 1865 state party platform: "Resolved, that we hold this to be a Government of White People, made and to be perpetuated for the exclusive political benefit of the White Race, and in accordance with the constant adjudication of the United States Supreme Court, that the people of African descent cannot be considered as citizens of the United States, and that there can in no event nor under any circumstances be any equality between the white and other Races." They opposed the goals articulated by Rufus King Cutler: "We have thirty to thirty-five thousand negro and colored voters in Louisiana, and about twenty-eight to thirty-thousand white voters. We could have all the negro men and the colored men to vote with the Union men, and that, with the disenfranchisement of the leading rebels, would give the ascendancy to the unionists, and I think they could sustain themselves...with sufficient military force to enforce these provisions, we could establish a government that would be substantial and we could sustain it after its establishment."

In addition, they argued legal technicalities: the elected chairman Howell had left the original convention before its conclusion and was, therefore, not considered a member, the constitution was accepted by the people, and the radicals, only 25 of whom were present at the convention of 1864, did not make up a majority of the original convention.

On July 27, Black supporters of the convention, including approximately 200 war veterans, met at the steps of the Mechanics Institute. They were stirred by speeches of abolitionist activists, most notably Anthony Paul Dostie and former Governor of Louisiana Michael Hahn. The men proposed a parade to the Mechanics Institute on the day of the convention to show their support.

The convention met at noon on July 30, but a lack of a quorum caused postponement to 1:30. When the convention members left the building, they were met by the black marchers with their marching band. On the corner of Common and Dryades streets, across from the Mechanics Institute, a group of armed whites awaited the black marchers. This group was composed largely of Democrats who opposed abolition; most were ex-Confederates who wanted to disrupt the convention and the threat to white supremacy the increasing political and economic power of blacks in the state represented.