The Prince-Bishopric of Warmia (; ) was a semi-independent ecclesiastical state, ruled by the incumbent ordinary of the Warmia see and comprising one third of the then diocesan area. The Warmia see was a Prussian diocese under the jurisdiction of the Archbishopric of Riga that was a protectorate of the Monastic state of the Teutonic Knights (1243–1464) and a protectorate and part of the Kingdom of Poland—later part of the Polish–Lithuanian Commonwealth (1464–1772), confirmed by the Peace of Thorn in 1466. The other two thirds of the diocese were under the secular rule of the Teutonic Knights until 1525 and Ducal Prussia thereafter, both entities also being a protectorate and part of Poland from 1466.
It was founded as the Bishopric of Ermland by William of Modena in 1243 in the territory of Prussia after it was conquered by the Teutonic Knights during the Northern Crusades. The diocesan cathedral chapter constituted in 1260. While in the 1280s the Teutonic Order succeeded to impose the simultaneous membership of all capitular canons in the Order in the other three Prussian bishoprics, Ermland's chapter maintained its independence. So Ermland's chapter was not subject to outside influence when electing its bishops. Thus the Golden Bull of Emperor Charles IV names the bishops as prince-bishops, a rank not awarded to the other three Prussian bishops (Culm (Chełmno), Pomesania, and Samland).
In 1440, most of the nobility and towns of Warmia joined the anti-Teutonic Prussian Confederation, upon the request of which the region was incorporated into the Kingdom of Poland in 1454, and in 1464, during the following Thirteen Years' War, the Prince-Bishopric of Warmia sided with Poland and officially recognized the overlordship of the Polish King. By the Second Peace of Thorn (1466) the Teutonic Knights renounced any claims to the prince-bishopric and recognized it as part of the Kingdom of Poland. It since formed part of the newly constituted Polish province of Royal Prussia, and after 1569 along with the province joined the Polish–Lithuanian Commonwealth, within which it was part of the larger Greater Poland Province, and Warmia's autonomy gradually faded, however, it flourished as an important religious, scientific and cultural center in Poland, thanks to such figures as Nicolaus Copernicus, Stanislaus Hosius, Marcin Kromer and Ignacy Krasicki.
After the First Partition of Poland in 1772, the Kingdom of Prussia annexed and secularised the prince-bishopric as a state. Its territory, Warmia, was incorporated into the newly formed Prussian province of East Prussia in 1773. Calvinist King Frederick II of Prussia confiscated the landed property of the Roman Catholic prince-bishopric and assigned it to the Kriegs- und Domänenkammer in Königsberg. In return he made up for the enormous debts of then Prince-Bishop Ignacy Krasicki.
By the Treaty of Warsaw (18 September 1773), King Frederick II guaranteed the free exercise of religion for the Catholics, so the religious body of the Roman Catholic diocese continued to exist, known since 1992 as the Roman Catholic Archdiocese of Warmia.
Within the State of the Teutonic Order
Along with Culm (Chełmno), Pomesania, and Samland (Sambia), Warmia was one of four dioceses in Prussia created in 1243 by the papal legate William of Modena. All four dioceses came under the rule of the appointed Archbishop of Prussia Albert Suerbeer who came from Cologne and was the former Archbishop of Armagh in Ireland. He chose Riga as his residence in 1251, which was confirmed by Pope Alexander IV in 1255. Heinrich of Strateich, the first elected Bishop of Warmia, was unable to claim his office, but in 1251 Anselm of Meissen entered the see of Warmia, which resided at Braunsberg (Braniewo) until it moved to Frauenburg (Frombork) in 1280 after attacks by heathen Old Prussians. The bishop ruled one-third of the bishopric as a secular ruler which was confirmed by the Golden Bull of 1356. The other two-thirds of the diocese were under the secular rule of the Teutonic Order.
The Bishops of Warmia generally defended their privileges and tried to put down all attempts to cut the prerogatives and the autonomy the bishopric enjoyed.
After the Battle of Grunwald in 1410, the Sambian and Warmian bishops paid homage to King Władysław II Jagiełło of Poland and Lithuania, a maneuver to protect the territory from complete destruction.