Glencorse is a parish of Midlothian, Scotland, lying 7 miles (11 kilometres) south of Edinburgh. It is bounded on the north-west by the former parish of Colinton now within the City of Edinburgh, to the north and west by Lasswade and to the south and west by Penicuik.

The parish is in the northern part of the Penicuik and District Community Council area and includes the village of Auchendinny near its eastern boundary.

The parish is traversed from west to east by Glencorse Burn, part of whose valley contains Glencorse Reservoir, which was formed in 1819–28 by damming the burn's glen with a huge embankment 40 m (130 ft) high. The reservoir is a source of Edinburgh's water supply. Also in the parish are Glencorse Barracks, Bush House, Glencorse House, Woodhouselee and Easter Howgate. The northern part of the parish includes some of the Pentland Hills and the highest point within the parish is Turnhouse Hill (428 m or 1,403 ft).

Historical records of the parish before 1878 spell it in various different ways - Glencors, Glencrosss, Glencrosse and Glencorse. After 1878 the last was normally used. There are several theories about the origin of the name Glencorse. One is that it comes form the Gaelic gleann - a glen and grosg or craig, a crossing - so a glen crossing the Pentland hills. George Chalmers in his book "Caledonia" claims the name comes from "a very remarkable cross, which has been erected in the vale of Glencross by pious hands." Yet another theory is that the name comes from a miraculous cross that appeared above Carnethy hill and encouraged the Scottish soldiers to defeat the English forces at the Battle of Roslin in 1303.

The parish is divided between two Scottish Parliamentary constituencies: Midlothian North and Musselburgh to the north of Glencorse burn and Midlothian South, Tweeddale and Lauderdale south of the burn.

The parish was originally named after St. Catherine, because of the chapel (St. Catherine-in-the-Hopes) dedicated to her which stood in the valley of Glencorse Burn (now submerged under a reservoir) and is mentioned in a record dated 1230. The parish of Glencorse was formed sometime before 1560 with the chapel of St. Catherine-in-the-Hopes as its church. Over the next century the parish was variously combined with the parishes of Dalkeith, Lasswade, Penicuik, Pentlands and the Bishopric of Edinburgh (In 1616, for example, it included the ancient parishes of Pentland and Penicuik, the northern part being taken from the parish of Pentland [Barony of Fulford] and the southern part from Penicuik). In March 1647, Glencorse was re-created by act of Parliament as a separate Parish with its own church building near the centre of the parish.

The Battle of Rullion Green between Covenanters and Scottish Government troops took place in 1666 in Glencorse Parish. The Covenanters were defeated.

The 1647 parish church was rebuilt. It was repaired and rebuilt several times from 1661 to 1691 and then mostly destroyed in a fire in 1697. The church (now known as the Glencorse Old Kirk) was rebuilt in 1699 with the north and south aisles being added and accommodation for 180 people. A tower and belfry were added to the church during renovations in 1811. The author Robert Louis Stevenson attended services in the church in his youth when he lived in Swanston village nearby and once described the Old Kirk as the "most delightful place on earth." Stevenson's letters to his friends often mention the Old Kirk and he used his memories as models for the churches in his books "A Lowder Sabbath Morning," "Weir of Hermiston" and "The Body Snatcher." Locally in Penicuik and Midlothian the Glencorse kirkyard was nicknamed "the Pirates Graveyard" due to the skull and crossbones on the graves despite this being a common feature on many graves.