The Royal Ulster Constabulary (RUC) was the police force in Northern Ireland from 1922 to 2001. It was founded on 1 June 1922 as a successor to the Royal Irish Constabulary (RIC) following the partition of Ireland. At its peak the force had around 8,500 officers, with a further 4,500 who were members of the RUC Reserve.

The RUC policed Northern Ireland from the aftermath of the Irish War of Independence until after the turn of the 21st century, and played a major role in the Troubles between the 1960s and the 1990s. Due to the threat from the Provisional Irish Republican Army (IRA), who saw the RUC as enforcing British rule, the force was heavily armed and militarised. Officers routinely carried submachine guns and assault rifles, travelled in armoured vehicles, and were based in heavily-fortified police stations. It was the first police force to use rubber and plastic bullets for riot control.

The RUC's membership was overwhelmingly Protestant, leading to accusations by sections of the Catholic and Irish nationalist minority of one-sided policing and sectarianism. Officers were also accused of police brutality as well as collusion with loyalist paramilitaries. Conversely, it was praised as one of the most professional police forces in the world by British security forces. During the Troubles, 319 RUC officers were killed and almost 9,000 injured in paramilitary assassinations or attacks, mostly by the IRA, which made the RUC the most dangerous police force in the world in which to serve by 1983. In the same period, the RUC killed 55 people, 28 of whom were civilians. In 2000, the RUC was awarded the George Cross for bravery.

The RUC was superseded by the Police Service of Northern Ireland (PSNI) in 2001, as mandated by the final version of the Police (Northern Ireland) Act 2000. Allegations regarding collusion prompted several inquiries, the most recent of which was authored by Police Ombudsman Nuala O'Loan in 2007. The report identified police, CID and Special Branch collusion with loyalist terrorists, but no member of the RUC has been charged or convicted of any criminal acts as a result of these inquiries. O'Loan stated in her conclusions that there was no reason to believe the findings of the investigation were isolated incidents.