The bishopric of Speyer belonged to the Upper Rhenish Circle of the Holy Roman Empire. One of the smallest principalities of the Holy Roman Empire, it consisted of more than half a dozen separate enclaves totalling about 28 German square miles (about 1540 km²) on both sides of the Rhine. It included the towns of Bruchsal (on the right bank) as well as Deidesheim, Herxheim bei Landau, and Lauterburg (on the left bank). Around 1800 the bishopric included about 55,000 people.
A diocese of Speyer has possibly existed since the 3rd or 4th century. It was first mentioned in historical documents in 614. Up to 748 it was a suffragant bishopric of the archdiocese of Trier, and from then until the secularisation of the prince-bishopric in 1803, of the archdiocese of Mainz.
The history of the Bishopric of Speyer began at the latest in the late 7th century when the bishop of Speyer received royal domains in the neighboring Speyergau. In the 10th and 11th centuries, the diocese received additional lands, including gifts by emperor Otto I. In 1030 the building of the cathedral was begun. In 1061 the cathedral was consecrated. In 1086 emperor Henry IV granted the bishopric the remaining parts of the county of Speyergau.
From 1111 the citizens of the city of Speyer began to increasingly loosen their bonds to the rulership of the bishop. In 1230 a Bürgermeister was mentioned for the first time. In 1294 Speyer became a Free Imperial City. The bishop moved his palace in 1371 to Udenheim. At the beginning of the 17th century bishop Philipp Christoph von Sötern expanded the fortress of Philippsburg. The prince-bishops reigned from there from 1371 to 1723. Afterwards the prince-bishop moved his seat to Bruchsal.
From 1681 to 1697, at the end of the War of the Grand Alliance, the fortress of Philippsburg on the left-bank went to France. In 1801/1802, the remaining left-bank territories of Speyer were conquered by French troops in the course of the French Revolution. The right-bank territories went to margraves of Baden.
This ended the secular responsibilities of the bishop of Speyer. The secularized bishopric continued ecclesiastically as the Diocese of Speyer. Finally, the French part of the former prince-bishopric was divided between Bavaria and Hesse Darmstadt in 1815.