The Loughinisland massacre took place on 18 June 1994 in the small village of Loughinisland, County Down, Northern Ireland. Members of the Ulster Volunteer Force (UVF), a loyalist paramilitary group, burst into a pub with assault rifles and fired on the customers, killing six civilians and wounding five. The pub was targeted because it was frequented mainly by Catholics, and was crowded with people watching the Republic of Ireland play against Italy in the 1994 FIFA World Cup. It is thus sometimes called the "World Cup massacre". The UVF claimed the attack was retaliation for the killing of three UVF members by the Irish National Liberation Army (INLA).

There have been allegations that police (Royal Ulster Constabulary) double agents or informers in the UVF were linked to the massacre and that police protected those informers by destroying evidence and failing to carry out a proper investigation. At the request of the victims' families, the Police Ombudsman investigated the police. In 2011 the Ombudsman concluded that there were major failings in the police investigation, but no evidence that police colluded with the UVF. The Ombudsman did not investigate the role of informers and the report was branded a whitewash. The Ombudsman's own investigators demanded to be disassociated from it. The report was quashed, the Ombudsman replaced and a new inquiry ordered.

In 2016, a new Ombudsman report concluded that there had been collusion between the police and the UVF, and that the investigation was undermined by the wish to protect informers, but found no evidence police had foreknowledge of the attack. Two documentary films about the massacre, Ceasefire Massacre and No Stone Unturned, were released in 2014 and 2017 respectively. The latter named the main suspects, one of whom was a British soldier, and claimed that one of the killers was an informer.