Slieve Gullion (from Irish Sliabh gCuillinn, meaning 'hill of the steep slope' or Sliabh Cuilinn, "Culann's mountain") is a mountain in the south of County Armagh, Northern Ireland. The mountain is the heart of the Ring of Gullion and is the highest point in the county, with an elevation of 573 metres (1,880 ft). At the summit is a small lake and two ancient burial cairns, one of which is the highest surviving passage grave in Ireland. Slieve Gullion appears in Irish mythology, where it is associated with the Cailleach and the heroes Fionn mac Cumhaill and Cú Chulainn. It dominates the countryside around it, offering views as far away as County Antrim, Dublin Bay and County Wicklow on a clear day. Slieve Gullion Forest Park is on its eastern slope.

Villages around Slieve Gullion include Meigh, Drumintee, Forkhill, Mullaghbawn and Lislea. The mountain gives its name to the surrounding countryside, and is the name of an electoral area within Newry, Mourne and Down District Council.

Slieve Gullion is a steep-sided mountain with a flat top and a height of 573 metres (1,880 ft). It is the eroded remains of a Paleocene volcanic complex. It is surrounded by a ring dike known as the Ring of Gullion, a designated Area of Outstanding Natural Beauty (AONB). Slieve Gullion has been shaped by glaciation and exhibits a classic 'crag and tail' glacial feature. The 'tail', made up of glacial deposits, points south, ending at Drumintee. The geological formation was the first ring dike to be mapped, although its significance was not understood until similar structures had been described from Scotland. The rocks of the area are complex and have featured in international geological debate since the 1950s. The site has attracted geologists from all over the world and featured in theories to explain the unusual rock relationships. Some of these theories have now become an accepted part of geological science.

Much of Slieve Gullion is covered with forest, heather, or raw stone, while 612 hectares of dry heath on the mountain has been designated a Special Area of Conservation, an Environmentally Sensitive Area, and an Area of Outstanding Natural Beauty. In July 2006, some areas of gorse were destroyed by a wildfire which may have been started deliberately.

Traces of fields on the mountain's poor soil from farming in earlier times can still be seen. There is also evidence of past quarrying.

There are two burial cairns on top of the hill, on either side of a small lake. The southern one is a large passage tomb, the highest surviving passage tomb in Ireland. In 1961, a team of archaeologists explored the site and set up a 30-person camp near the summit. The tomb's cairn is 30 m (98 ft) wide and 5 m (16 ft) high. The chamber inside is 3.6 m (12 ft) wide, with a corbelled roof up to 4.3 m (14 ft) high. It contained three large blocks of stone seemingly used as basins, and fragments of human bone. Some bits of worked flint and a barbed-end arrowhead were also found, "the meager remnants that survived the centuries of tomb raiding". The entrance is aligned with the setting sun on the winter solstice. Radiocarbon dating suggests it was built c.3500–2900 BCE. The smaller cairn to the north of the lake was built later, perhaps during the early Bronze Age (around 2000 BC). It contains two cist burials, with one containing bits of burnt bone; likely the remains of a single adult.

The two cairns were disturbed by American soldiers training there during World War II. Irish folklore holds that it is bad luck to damage or disrespect such tombs and that doing so could bring a curse. Today they are historic monuments protected by law. In recent years, volunteers have helped to repair the burial cairns under the supervision of an archaeologist.

Around 1680, religious persecution of local Irish Catholics heated up in reaction to Titus Oates' claims of a non-existent Catholic conspiracy aimed at assassinating King Charles II of England and massacring the Protestants of the British Isles. As a result, a Catholic priest named Fr. Mac Aidghalle was murdered while saying mass at a mass rock that still stands atop Slieve Gullion. The perpetrators were a company of redcoats under the command of a priest hunter named Turner. Redmond O'Hanlon, the outlawed but de facto Chief of the Name of Clan O'Hanlon and leading local rapparee, is said in local oral tradition to have avenged the murdered priest and in so doing to have sealed his own fate.