The Ulster Defence Regiment (UDR) was an infantry regiment of the British Army established in 1970, with a comparatively short existence ending in 1992. Raised through public appeal, newspaper and television advertisements, their official role was the "defence of life or property in Northern Ireland against armed attack or sabotage" but unlike troops from Great Britain they were never used for "crowd control or riot duties in cities". At the time the UDR was the largest infantry regiment in the British Army, formed with seven battalions plus another four added within two years.

It consisted mostly of part-time volunteers until 1976, when a full-time cadre was added. Recruiting in Northern Ireland at a time of intercommunal strife, some of its (mostly Ulster Protestant) members were involved in sectarian killings. The regiment was originally intended to more accurately reflect the demographics of Northern Ireland, and began in 1970 with Catholic recruits accounting for 16% of its soldiers; but by the end of 1972, after the introduction of internment this had dropped to around 3%. It is doubtful if any other unit of the British Army has ever come under the same sustained criticism as the UDR.

Uniquely in the British Army, the regiment was on continuous active service throughout its 22 years of service. It was also the first infantry regiment of the British Army to fully incorporate women into its structure. In 1992, the UDR was amalgamated with the Royal Irish Rangers to form the Royal Irish Regiment. In 2006, the regiment was retroactively awarded the Conspicuous Gallantry Cross.

The UDR was raised in 1970, soon after the beginning of the Northern Ireland "Troubles". Before then, the main security forces were the Royal Ulster Constabulary (RUC) and the Ulster Special Constabulary (USC), otherwise known as the "B Specials". Catholics were reluctant to join what they saw as unionist militias that lacked impartiality leading to the forces becoming almost entirely Protestant.

Large scale intercommunal rioting in 1969 stretched police resources in Northern Ireland, so the British Army was deployed to assist the police. On 28 August 1969 security in Northern Ireland, including the USC, was put under the direct control of the General Officer Commanding in Northern Ireland, General Ian Freeland.

The USC, which had no training in riot control, was mobilised to assist the RUC. A catalogue of incidents ensued, such as Specials from Tynan shooting dead an unarmed civil rights demonstrator in Armagh on 14 August 1969. While the Northern Ireland cabinet remained supportive of the USC, it was put to them at a London meeting on 19 August that disbanding the USC was top of the British government's agenda.

The Hunt Report commissioned by the Government of Northern Ireland published on 3 October 1969, recommended that the RUC "should be relieved of all duties of a military nature as soon as possible". Further; a "locally recruited part-time force, under the control of the G.O.C., Northern Ireland, should be raised" ... and that it "together with the police volunteer reserve, should replace the Ulster Special Constabulary". The new force was to be "impartial in every sense" and "remove the responsibility of military-style operations from the police".

The British government accepted the findings of the Hunt Report and published a Bill and white paper on 12 November 1969 to begin the process of establishing the UDR. Parliamentary debate in Westminster highlighted concerns that members of the USC were to be allowed to join the new force.