The Diocese of Montalcino (Latin: Dioecesis Ilcinensis) was a Roman Catholic diocese located in the town of Montalcino to the west of Pienza, close to the Crete Senesi in Val d'Orcia in Tuscany, Italy. In 1986, it was suppressed and united with the Diocese of Colle di Val d'Elsa and the Archdiocese of Siena to form the Archdiocese of Siena-Colle di Val d'Elsa-Montalcino.
On 13 August 1462, Pope Pius II established the Diocese of Montalcino, drawing its territory from the Diocese of Arezzo, the Diocese of Chiusi, and the Diocese of Grosseto The new dioceses were removed from all jurisdiction of the metropolitan archbishop of Siena, and made directly subject to the Holy See (Papacy). If a bishop wished, however, he could submit a case to the archbishop, who was authorized to take cognizance of it. The bishop of Pienza and of Montalcino was not obligated to attend the provincial synods of Siena, though he could do so if he wished.
In 1528, Pope Clement VII separated the two dioceses of Montalcino and Pienza into two episcopal jurisdictions with two bishops.
On 15 June 1772, Montalcino gained territory from the Diocese of Chiusi and from the Diocese of Pienza
The cathedral was originally a collegiate church, dedicated to the Holy Saviour (San Salvatore)
The cathedral was staffed and administered by a Chapter, composed of four dignities (the Archdeacon, the Archpriest, the Provost, and the Primicerius) and nine Canons. There were in addition four honorary Canons
The Second Vatican Council, in order to ensure that all Catholics received proper spiritual attention, decreed the reorganization of the diocesan structure of Italy and the consolidation of small and struggling dioceses.
In 1980, the diocese of Montalcino claimed a Catholic population of 24,500 persons. Colle di Val d'Elsa had slightly over 60,000.
On 18 February 1984, the Vatican and the Italian State signed a new and revised concordat. Based on the revisions, a set of Normae was issued on 15 November 1984, which was accompanied in the next year, on 3 June 1985, by enabling legislation. According to the agreement, the practice of having one bishop govern two separate dioceses at the same time, aeque personaliter, was abolished. This made the combining of Montalcino and Colle di Val d'Elsa under one bishop infeasible. Instead, the Vatican continued consultations which had begun under Pope John XXIII for the merging of small dioceses, especially those with personnel and financial problems, into one combined diocese. On 30 September 1986, Pope John Paul II ordered that the dioceses of Montalcino and Colle be merged with the diocese of Siena, into one diocese with one bishop, with the Latin title Archidioecesis Senensis-Collensis-Ilcinensis. The seat of the diocese was to be in Siena, and the cathedral of Siena was to serve as the cathedral of the merged dioceses. The cathedrals in Montalcino and Colle were to become co-cathedrals, and the cathedral Chapters were each to be a Capitulum Concathedralis. There was to be only one diocesan Tribunal, in Siena, and likewise one seminary, one College of Consultors, and one Priests' Council. The territory of the new diocese was to include the territory of the former dioceses of Montalcino and of Colle.
Erected: 13 August 1462<br> Latin Name: Pientia et Mons Ilcinus
1528: Split into the Diocese of Montalcino and the Diocese of Pienza<br> Latin Name: Ilcinensis