The Diocese of Vác, (Latin: Dioecesis Vaciensis) is a Latin Church ecclesiastical territory or diocese of the Catholic church in Hungary, with its seat in Vác. The diocese was created in 1008 by St. Stephen, the first King of Hungary. Originally known as the "Diocese of Waitzen" in German, it is a suffragan diocese in the ecclesiastical province of the Archdiocese of Eger. The current bishop is Zsolt Marton, who was appointed in 2019.

Its first bishops were Clement, Lazarus, and Aaron. Lazarus is believed to have been bishop from 1075 to 1077; Stephen was known to have been bishop in 1102. Beginning with Marcellus (1105–19), the series of bishops is uninterrupted. Particularly notable early bishops of Vác include: John de Surdis (1363–73), ambassador of King Louis I to Italy in 1369, later on Archbishop of Esztergom; Vincent Szilassy (1450–73), a member of the embassy which brought the newly elected King Matthias Corvinus from Prague to Vác; Wladislaw Szalkai (1514–23), chancellor of King Louis II and afterwards Archbishop of Esztergom; Martinus Pethe (1582–86), transferred to Kalocsa.

Later important bishops include Sigismund Kolonits (1709–16), transferred to Vienna, and first Archbishop of Vienna; Count Michael Friedrich von Althann (1718–34), sent as viceroy to Sicily by Emperor Charles VI, and afterwards cardinal; Count Christopher Migazzi, cardinal and Archbishop of Vienna, twice Bishop of Vác (1756–57); 1762–82); Augustinus Roskoványi (1851–59), an eminent theological writer, transferred to Nyitra in 1859. Roskoványi was succeeded by Anthony Peitler, 1859–85, who founded the library at Vác. In 1900 Count Charles Csáky became bishop.

In 1514, when the Ottoman Turks conquered Vác, the cathedral chapter ceased to exist, but was re-established in 1700.

In the early 20th century, the diocese included parts of the counties of Nógrád, Pest, Csongrád and Jász-Nagykun-Szolnok, divided into three archdeaconries and nineteen vice-archdeaconries. Within the diocese were five titular abbeys, four provostships and six titular provostships. The chapter had twelve canons and six titular canons. The number of parishes was 123; that of the clergy, 266. The right of patronage was exercised by 44 patrons. The diocese included 7 monasteries and 12 nunneries, with altogether 232 inmates. The Catholic population was 757 827.

• Diocese of Vác; catholic-hierarchy.org

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 This article incorporates text from a publication now in the public domain: Herbermann, Charles, ed. (1913). "Diocese of Waitzen". Catholic Encyclopedia. New York: Robert Appleton Company.

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