The Diocese of Guardialfiera (Latin Dioecesis Guardiensis Alpheriae) or Diocese of Guardia was a Roman Catholic diocese in Italy. The diocese was established in the second half of the 11th century, with seat of the diocese was located in the city of Guardialfiera in the Province of Campobasso in the region of Molise. In 1818, the diocese was suppressed, and its ecclesiastical territory was assigned to the Diocese of Termoli.
It has been a titular see since 1968.
The earliest known bishop of Guardia was Petrus, who is first recorded in 1071. The diocese is first named in the confirmation of the privileges of the archbishops of Benevento made by Pope Anastasius IV on 22 September 1153, in which the suffragans of the metropolitan are listed.
Following the extinction of the Napoleonic Kingdom of Italy, the Congress of Vienna authorized the restoration of the Papal States and the Kingdom of Naples. Since the French occupation had seen the abolition of many Church institutions in the Kingdom, as well as the confiscation of much Church property and resources, it was imperative that Pope Pius VII and King Ferdinand IV reach agreement on restoration and restitution. Ferdinand demanded the suppression of fifty dioceses.
A concordat was finally signed on 16 February 1818, and ratified by Pius VII on 25 February 1818. Ferdinand issued the concordat as a law on 21 March 1818. On 27 June 1818, Pius VII issued the bull De Ulteriore, in which the ecclesiastical province of Benevento was restored, including it suffragans, among them the diocese of Termoli. The decision was also made to suppress permanently the diocese of Guardialfiera, and to incorporate its territory into the diocese of Termoli.
In 1968, the title of "Bishop of Guardialfiera" (though not the diocese itself) was restored as the Titular Episcopal See of Guardialfiera. On 28 April 1969, Pope Paul VI announced a large number of appointments, including that of a titular bishop of Guardialfiera, Bishop Ramón Sanahuja y Marcé.