The Prince-Bishopric of Brandenburg was an imperial estate of the Holy Roman Empire for some time, probably starting about 1161/1165. However, the Brandenburg bishops never managed to gain control over a significant territory, being overshadowed by the Margraviate of Brandenburg, which was originally seated in the same city. Chapter and cathedral, surrounded by further ecclesiastical institutions, were located on the Dominsel (Cathedral Island), which formed a prince-episcopal cathedral immunity district (Domfreiheit), distinct from the city of Brandenburg. Only in 1929 the - meanwhile former - immunity district was incorporated into the city itself.
The foundation charter of the Brandenburg diocese is dated 1 October 948, though the actual founding date remained disputed among historians. The medieval chronicler Thietmar of Merseburg mentions the year 938; the bishopric may also have been established in the course of the partition of the vast Marca Geronis and the emergence of the Northern March after Margrave Gero's death in 965. With the foundation, King Otto (Holy Roman Emperor from 962) aimed at the Christianization of the Polabian Slavs (Wends) and the incorporation of their territory into the East Frankish realm.
Brandenburg was originally a suffragan of the Archbishopric of Mainz, but in 968 it came under the jurisdiction of the Magdeburg archbishops. The Great Slav Rising of 983 practically annihilated it, when revolting Lutici tribes conquered Brandenburg and the neighbouring Bishopric of Havelberg. Brandenburg bishops continued to be appointed, but they were merely titular, residing in Magdeburg or acting as auxiliary bishops in the western territories of the Empire. Not until the final subjugation of the Wends in the 12th century by Margrave Albert the Bear, the German eastward settlement (Ostsiedlung) in the diocesan region revived the bishopric.
Bishop Wigers of Brandenburg (acting 1138–60), an adherent of Norbert of Xanten, was the first of a series of bishops of the Premonstratensian Order, which chose the occupants of the episcopal see until 1447; in that year a bull of Pope Nicholas V gave the right of nomination to the Brandenburg elector, with whom the bishops stood in a close feudal relation. Bishop Wigers also established a Premonstratensian convent at Leitzkau (today part of Gommern, Saxony-Anhalt). Probably at the request of the Hevellian prince Pribislav-Henry, he established another convent at the Slavic Parduin settlement in present-day Brandenburg an der Havel, which became the nucleus of the revived Brandenburg cathedral chapter. The incorporation into the Premonstratensian Order was confirmed by Pope Clement III in 1188.
Ziesar Castle, now a museum also showing the history of the Prince-Bishopric of Brandenburg
As rulers of imperial immediacy, regnant in a, however, dispersed territory partitioned into the four bailiwicks (German: Ämter) of Brandenburg/Havel, Ketzin, Teltow and Ziesar. The prince-bishops from the early 14th century onwards resided in their fortress in Ziesar on the road to Magdeburg. The last actual bishop was Matthias von Jagow (d. 1544), who took the side of the Protestant Reformation, married, and in every way furthered the undertakings of the Hohenzollern elector Joachim II.
There were two more nominal bishops, but on the petition of the latter of these, the electoral prince John George of Brandenburg appointed in 1560, the secularisation of the bishopric was undertaken and finally accomplished in 1571, in spite of legal proceedings to reassert the imperial immediacy of the prince-bishopric within the Empire and so to likewise preserve the diocese, which dragged on into the 17th century.
