The Diocese of Castello, originally the Diocese of Olivolo, is a former Roman Catholic diocese that was based on the city of Venice in Italy. It was established in 774, covering the islands that are now occupied by Venice. Throughout its existence there was tension between the diocese, the Patriarchate of Grado to which it was nominally subordinate, and the Doge of Venice. Eventually in 1451 the diocese and the patriarchate were merged to form the Archdiocese of Venice.

This is a list of the Patriarchs of Grado.