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.

The Ulster Defence Association emerged from a series of meetings during the middle of 1971 of loyalist "vigilante" groups called "defence associations". The largest of these were the Shankill and Woodvale Defence Associations, with other groups based in East Belfast, Lower Shankill and Roden Street.

The first meeting, in September 1971, was chaired by Billy Hull, with Alan Moon of the lower Shankill as its vice-chair. Moon was quickly replaced by Jim Anderson. Moon, who had become reluctant to be involved in vigilantism since the group's formation, willingly stepped aside and ended his association with the UDA soon afterwards. The structure of this new movement soon took shape with a thirteen-man Security Council established in January 1972 as a reaction to a Provisional IRA bomb the previous month at the Balmoral furniture showroom on the Shankill which killed four people including two infants.

By this point, Charles Harding Smith had become the group's leader, with former Royal Army Ordnance Corps soldier Davy Fogel as his second-in-command, who trained the new recruits in military tactics, the use of guns, and unarmed combat. Its most prominent early spokesperson was Tommy Herron; however, Andy Tyrie would emerge as leader soon after. Its original motto was Cedenta Arma Togae ("Law before violence") and it was a legal organisation until it was banned by the British government on 10 August 1992. Under Smith's command, the UDA was organised along paramilitary lines into battalions, companies, platoons and sections. The organisation drew more members, becoming the largest loyalist paramilitary organisation in Northern Ireland. Unlike its principal rival, the Ulster Volunteer Force (UVF), the UDA was legal.

In April 1972, the organisation's leader, Charles Harding Smith and leading UDA member John White were arrested in London for gun-trafficking. A temporary de facto leadership assumed control and Anderson became the acting chairman of the UDA.

At the end of May 1972, Fogel, by then the leader of B Company and Harding Smith's second-in-command, erected the first UDA roadblocks and street barricades, making Woodvale, the area under Fogel's command, a no-go area. The operation attracted a great deal of media and press coverage, resulting in much publicity for the UDA. British Army troops under the command of Major-General Robert Ford were sent to the area, where a stand-off with the UDA ensued. Leading UDA figures eventually entered into street negotiations with senior Army officers, where it was eventually agreed that the UDA could erect small temporary barriers in Loyalist neighbourhoods. That summer, the UDA marched through the streets of central Belfast in a massive demonstration of strength.

In December 1972, Harding Smith and White were acquitted and returned to Belfast. Immediately after their return, a fierce power struggle ensued after Harding Smith declared to his associates: "I'm the boss. I take orders from no one". Fogel was promptly ousted from the B Company command, while the formidable East Belfast brigadier, Tommy Herron, appeared on the scene to challenge Harding Smith's leadership. Anderson became joint chairman of the UDA with Harding Smith. The struggle that ensued between Harding Smith and Herron overshadowed the Inner Council and during the height of the feud Anderson often had to call a register at its meetings, so poor were the turnouts. Herron and Anderson became linked and the East Belfast brigadier took to styling himself as deputy leader to Anderson, whom he treated as sole chairman.