St Giles' Cathedral (Scottish Gaelic: Cathair-eaglais Naomh Giles), also known as High Kirk of Edinburgh, is one of three cathedrals in Edinburgh, Scotland. The existing building was begun in the 14th century and extended until the early 16th century; significant alterations were undertaken in the 19th and 20th centuries, including the addition of the Thistle Chapel. St Giles' is closely associated with many events and figures in Scottish history, including John Knox, who served as the church's minister after the Scottish Reformation. The cathedral is administered by the Church of Scotland.

It was probably founded in the 12th century and dedicated to Saint Giles. The church was elevated to collegiate status by Pope Paul II in 1467. In 1559, the church became Protestant with John Knox, the foremost figure of the Scottish Reformation, as its minister. After the Reformation, St Giles' was internally partitioned to serve multiple congregations as well as secular purposes, such as a prison and as a meeting place for the Parliament of Scotland. In 1633, Charles I made St Giles' the cathedral of the newly created Diocese of Edinburgh. Charles' attempt to impose doctrinal changes on the presbyterian Scottish Kirk, including a Prayer Book causing a riot in St Giles' on 23 July 1637, which precipitated the formation of the Covenanters and the beginnings of the Wars of the Three Kingdoms. St Giles' role in the Scottish Reformation and the Covenanters' Rebellion has led to its being called "the Mother Church of World Presbyterianism".

St Giles' is one of Scotland's most important medieval parish church buildings. The first church of St Giles' was a small Romanesque building of which only fragments remain. In the 14th century, this was replaced by the current building which was enlarged between the late 14th and early 16th centuries. The church was altered between 1829 and 1833 by William Burn and restored between 1872 and 1883 by William Hay with the support of William Chambers. Chambers hoped to make St Giles' a "Westminster Abbey for Scotland" by enriching the church and adding memorials to notable Scots. Between 1909 and 1911, the Thistle Chapel, designed by Robert Lorimer, was added to the church.

Since the medieval period, St Giles' has been the site of nationally important events and services; the services of the Order of the Thistle take place there. Alongside housing an active congregation, the church is one of Scotland's most popular visitor sites: it attracted over a million visitors in 2018.

Saint Giles is the patron saint of lepers. Though chiefly associated with the Abbey of Saint-Gilles in modern-day France, he was a popular saint in medieval Scotland. The church was first possessed by the monks of the Order of St Lazarus, who ministered among lepers; if David I or Alexander I is the church's founder, the dedication may be connected to their sister Matilda, who founded St Giles in the Fields.

Prior to the Reformation, St Giles' was the only parish church in Edinburgh and some contemporary writers, such as Jean Froissart, refer simply to the "church of Edinburgh". From its elevation to collegiate status in 1467 until the Reformation, the church's full title was "the Collegiate Church of St Giles of Edinburgh". Even after the Reformation, the church is attested as "the college kirk of Sanct Geill". The charter of 1633 raising St Giles' to a cathedral records its common name as "Saint Giles' Kirk".

St Giles' held cathedral status between 1633 and 1638 and again between 1661 and 1689 during periods of episcopacy within the Church of Scotland. Since 1689, the Church of Scotland, as a Presbyterian church, has had no bishops and, therefore, no cathedrals. St Giles' is one of a number of former cathedrals in the Church of Scotland – such as Glasgow Cathedral or Dunblane Cathedral – that retain their title despite having lost this status. Since the church's initial elevation to cathedral status, the building as a whole has generally been called St Giles' Cathedral, St Giles' Kirk or Church, or simply St Giles'.

The title "High Kirk" is briefly attested during the reign of James VI as referring to the whole building. A 1625 order of the Privy Council of Scotland refers to the Great Kirk congregation, which was then meeting in St Giles', as the "High Kirk". The title fell out of use until reapplied in the late 18th century to the East (or New) Kirk, the most prominent of the four congregations then meeting in the church. Since 1883, the High Kirk congregation has occupied the entire building.