Belfast's Bloody Sunday occurs with 20 killings, at least 100 wounded and 200 homes destroyed during rioting and gun battles in Belfast, Northern Ireland.

Bloody Sunday or Belfast's Bloody Sunday was a day of violence in Belfast, Northern Ireland on 10 July 1921, during the Irish War of Independence. The violence erupted one day before a truce began, which ended the war in most of Ireland. With the truce nearing, police launched a raid against republicans, but were ambushed by the Irish Republican Army (IRA) and an officer was killed. In retaliation, Protestant loyalists attacked Catholic enclaves in west Belfast, burning homes and businesses. This sparked rioting and gun battles between Protestants and Catholics, including paramilitaries. There were also gun battles between republicans/nationalists and the police, and some police patrols fired indiscriminately at Catholic civilians. Seventeen people were killed or fatally wounded on 10 July, and a further three were killed or fatally wounded before the truce began at noon on 11 July. At least 100 people were wounded. About 200 houses were destroyed or badly damaged, most of them Catholic homes, leaving 1,000 people homeless. See: The Troubles in Ulster (1920–1922).

Belfast is the capital city and principal port of Northern Ireland, standing on the banks of the River Lagan and connected to the open sea through Belfast Lough and the North Channel. It is the second-largest city in Ireland, with an estimated population of 348,005 in 2022, and a metropolitan area population of 671,559.

July 10 is the 191st day of the year in the Gregorian calendar; 174 days remain until the end of the year.