The former Norwegian Catholic diocese of Hamar existed from 1152 to the Protestant Reformation. The see was at Hamar, and the diocese included the (modern) counties of Hedmark (except Solør, Odalen and the northern part of Østerdalen), Oppland (except Valdres), and the middle part of Buskerud (the modern municipalitites of Flesberg, Hole, Krødsherad, Modum, Nore og Uvdal, Ringerike, Rollag and Sigdal). It also included some parts of Telemark (the modern municipalitites of Hjartdal, Notodden, Seljord, Tinn and Vinje).
It was formed in 1152 out of the diocese of Oslo, when Arnold, Bishop of Gardar, Greenland (1124–52), was appointed first Bishop of Hamar. He began to build the now ruined cathedral of Christ Church, which was completed about the time of Bishop Paul (1232–52).
Bishop Thorfinn (1278–82) was exiled and died at Ter Doest in Flanders. Bishop Jörund (1285–86) was transferred to the archdiocese of Trondhjem. A provincial council was held in 1380.
The last Catholic bishop, Mogens (1513–37), was taken prisoner in his castle at Hamar by Truid Ulfstand, a Danish noble, and sent to Antvorskov in Denmark, where he was held until his death in 1542. There were at Hamar a cathedral chapter with ten canons, a school, a Dominican Priory of St. Olaf, and a monastery of the Canons Regular of St. Anthony of Vienne.