The Ulster Defence Association (UDA) is an Ulster loyalist paramilitary group in Northern Ireland. It was formed in September 1971 as an umbrella group for various loyalist groups and undertook an armed campaign of almost 24 years as one of the participants of the Troubles. Its declared goal was to defend Ulster Protestant loyalist areas and to combat Irish republicanism, particularly the Provisional Irish Republican Army (IRA). In the 1970s, uniformed UDA members openly patrolled these areas armed with batons and held large marches and rallies. Within the UDA was a group tasked with launching paramilitary attacks that used the cover name Ulster Freedom Fighters (UFF) so that the UDA would not be outlawed. The British government proscribed the UFF as a terrorist group in November 1973, but the UDA itself was not proscribed until August 1992.

The UDA/UFF were responsible for more than 400 deaths. The vast majority of its victims were Irish Catholic civilians, killed at random, in what the group called retaliation for IRA actions or attacks on Protestants. High-profile attacks carried out by the group include the Top of the Hill bar shooting, the Milltown massacre, the Sean Graham's and James Murray's bookmakers' shootings, the Castlerock killings, killings of Paddy Wilson and Irene Andrews and the Greysteel massacre. Most of its attacks were in Northern Ireland, but from 1972 onward it also carried out bombings in the Republic of Ireland. The UDA/UFF declared a ceasefire in 1994 and ended its campaign in 2007, but some of its members have continued to engage in violence. The other main Loyalist paramilitary group during the conflict was the Ulster Volunteer Force (UVF). All three groups are proscribed organisations in the United Kingdom under the Terrorism Act 2000.