The Greysteel massacre was a mass shooting that took place on the evening of 30 October 1993 in Greysteel, County Londonderry, Northern Ireland. Members of the Ulster Defence Association (UDA), a loyalist paramilitary group, opened fire on civilians in a crowded pub during a Halloween party, killing eight and wounding nineteen. The pub was targeted because it was frequented by Catholics, though two of the victims were Protestant. The group claimed responsibility using their cover name "Ulster Freedom Fighters", saying the attack was revenge for the Shankill Road bombing by the Provisional IRA a week earlier. Four men were sentenced to life imprisonment for the massacre but were released in 2000 under the terms of the Good Friday Agreement.

On 23 October 1993, an IRA bomb prematurely exploded as the bombers were carrying it into a fishmongers on the Shankill Road, Belfast. The IRA's intended target was a meeting of UDA leaders, including brigadier Johnny Adair, which was to take place in a room above the shop. Unknown to the IRA, the meeting had been rescheduled. Eight Protestant civilians, a UDA member, and one of the IRA bombers were killed in the blast. This became known as the Shankill Road bombing.

The UDA launched several "revenge attacks" for the bombing. Later that day, they fatally shot a Catholic delivery driver after luring him to a bogus call at Vernon Court, Belfast. On 26 October, the UDA shot dead another two Catholic civilians and wounded five in an attack at the Council Depot at Kennedy Way, Belfast.

The order for the attack came from the UDA leadership, and it is believed Greysteel was chosen partly because it was well away from Belfast, where security force activity was intense after the Shankill bombing. Those involved in planning and organising it included Billy McFarland, 'Brigadier' of the UDA's North Antrim & Londonderry Brigade. Stephen Irwin, Geoffrey Deeney, and Torrens Knight, all members of the brigade, were to carry out the shooting. The gunmen were first briefed on the plans for the massacre on 27 October in an office owned by the Ulster Democratic Party at Bond's Place, Derry.

Before the massacre, the gunmen went to the pub to familiarise themselves with the layout and choose the best positions from which to shoot. Knight made Irwin and Deeney rehearse the shooting in the office at Bond's Place. The gunmen would drive to the pub in an Opel Kadett with UDA member Brian McNeill driving a 'scout car' in front.

Just before 10 pm on Saturday 30 October, the three gunmen, wearing blue boiler suits and balaclavas, entered the Rising Sun Bar in Greysteel. There were at least 70 people inside attending a Halloween party and at first some believed the men were playing a Halloween prank. Stephen Irwin yelled "trick or treat" as he opened fire with a vz. 58 assault rifle on the packed crowd in the lounge. He kept shooting until the magazine emptied, quickly reloaded, and continued shooting. Geoffrey Deeney opened fire with a 9mm handgun at a fleeing woman, but it jammed after one shot. Torrens Knight, armed with a shotgun, guarded the entrance while the shooting was taking place. There was panic and screaming as people scrambled for cover and women pleaded for mercy. The scene in the Rising Sun was described as "hell-like"; bodies lay everywhere and the lounge and dancefloor were covered with blood and broken glass.

The gunmen, laughing, then made their escape in the Opel Kadett driven by Knight. While driving away from Greysteel, the getaway car's wing mirror was hit by a police car speeding towards the scene. The gunmen drove to a pick-up point near Eglinton, where they met McNeill and burned the car.

Seven people were killed outright and nineteen were wounded, with another later dying of his wounds. The dead were Karen Thompson (19), Steven Mullan (20), Moira Duddy (59), Joseph McDermott (60), James Moore (81), John Moyne (50), John Burns (54), and Victor Montgomery (76). Six of those killed were Catholic civilians and two were Protestant civilians.