The See of Seckau was founded on 22 June 1218, then the third suffragan of the metropolitan diocese of Salzburg after Gurk (1072) and Chiemsee (1215), by Archbishop Eberhard von Regensberg with permission from Pope Honorius III.

Emperor Frederick II gave his consent on 26 October 1218; he conferred on the incumbent of the see the dignity of a Prince of the Roman Empire, though with no secular power.

The first bishop was Provost Karl von Friesach (1218–1230) who had his see at Seckau Abbey in Upper Styria; his diocese only comprised 13 parishes.

Most of the time, the Seckau bishops resided at Seggau Castle near Leibnitz and at Graz, they also served as vicars in the Duchy of Styria.

The diocese also operates a religious museum (Diözesanmuseum), housed in the former Jesuit University building in the Graz Old Town across from the cathedral and the Church of St. Catherine of Alexandria with the mausoleum of Emperor Ferdinand II.