The Archbishopric of Trier (Erzbistum Trier) was a Roman Catholic diocese in Germany, that existed from Carolingian times until the end of the Holy Roman Empire. Its suffragans were the dioceses of Metz, Toul and Verdun. Since the 9th century the Archbishops of Trier were simultaneously princes and since the 11th century prince electors of the empire. Their temporal territories were known as the Electorate of Trier (Kurfürstentum Trier or Kurtrier).
The bishops of Trier were already virtually independent territorial magnates in Merovingian times. In 772 Charlemagne granted Bishop Wiomad complete immunity from the jurisdiction of the ruling count for all the churches and monasteries, as well as villages and castles that belonged to the Church of St. Peter at Trier. In 816 Louis the Pious confirmed to Archbishop Hetto the privileges of protection and immunity granted by his father.
At the partition of the Carolingian empire at Verdun in 843, Trier fell to Lothair; at the partition of Lotharingia at Mersen in 870, it fell to the East Frankish kingdom, which developed into Germany. Archbishop Radbod received in 898 complete immunity from all taxes for the entire episcopal territory, granted by Zwentibold, the natural son of Emperor Arnulf of Carinthia, who reigned briefly as King of Lotharingia and, under great pressure from his independent nobles, desperately needed a powerful ally. The gift cemented the position of the archbishops as territorial lords in their own right. Following Zwentibold's assassination in 900, the handlers of the child-king Louis courted Radbold in their turn, granting him the district and city of Trier outright, and the right to have a mint — as much a symbol of independent authority as an economic tool — and to impose customs-duties. From the court of Charles the Simple he obtained the final right, that of election of the Bishop of Trier by the chapter, free of Imperial interference.
The last archbishop-elector removed to Koblenz in 1786. From 1795, the territories of the Archbishopric on the left bank of the Rhine — which is to say almost all of them — were under French occupation, and were annexed in 1801 and a separate bishopric established (later assuming control of the whole diocese in 1803). In 1803, what was left of the Archbishopric was secularized and annexed by the Princes of Nassau.