The Cathedral of St Andrew (often referred to as St Andrews Cathedral) is a ruined Roman Catholic cathedral in St Andrews, Fife, Scotland. It was built in 1158 and became the centre of the Medieval Catholic Church in Scotland as the seat of the Archdiocese of St Andrews and the Bishops and Archbishops of St Andrews. It fell into disuse and ruin after Catholic mass was outlawed during the 16th-century Scottish Reformation. It is currently a monument in the custody of Historic Scotland. The ruins indicate that the building was approximately 391 feet (over 100 metres) long, and is the largest church to have been built in Scotland.
Saint Regulus or Saint Rule (Old Gaelic: Riagal) was a legendary 4th century monk or bishop of Patras, Greece who in AD 345 is said to have fled to Scotland with the bones of Saint Andrew, and deposited them at St Andrews. His feast day in the Aberdeen Breviary is October 17.