St Magnus Cathedral is a Church of Scotland parish church in Kirkwall, the main town of Orkney, a group of islands off the north coast of mainland Scotland. Originally a Roman Catholic cathedral, it is the oldest cathedral in Scotland and the most northerly cathedral in the United Kingdom. A large edifice that dominates the skyline of Kirkwall, it is a fine example of Romanesque architecture with later Gothic additions, built when the islands were ruled by the Norse Earls of Orkney as a semi-autonomous part of the Kingdom of Norway. The building is today owned by Orkney Islands Council as successor of the burgh of Kirkwall as a result of an act of King James III of Scotland following Orkney's annexation by the Scottish Crown in 1468.
Construction began in 1137 and it was added to over the next 300 years. The first bishop of Orkney was William the Old and it was for Bishop William that the nearby Bishop's Palace was built. Before the Scottish Reformation, the cathedral was presided over by the Bishop of Orkney. Today, it is a parish church of the Church of Scotland (with a presbyterian system of Church governance). As of 2024, the congregation of St Magnus Cathedral is part of Orkney Islands Church of Scotland – a single Church of Scotland ecclesiastical parish. The cathedral is listed at Category A, the highest grade of listed building in Scotland.