| Jurisdiction: | Diocese |
| Alexandria–Cornwall | |
| Latin: | Dioecesis Alexandrina–Cornubiensis |
| Country: | Canada |
| Territory: | St. Lawrence Valley |
| Catholics: | 56,600 |
| Catholics Percent: | 62.2 |
| Denomination: | Catholic |
| Sui Iuris Church: | Latin Church |
| Rite: | Roman Rite |
| Established: | January 23, 1890 |
| Cathedral: | St. Finnan's Cathedral, Alexandria |
| Cocathedral: | Co-cathédrale de la Nativité, Cornwall |
| Patron: | St. Finnan |
| Bishop: | Terrence Prendergast |
The Roman Catholic Diocese of Alexandria–Cornwall (Latin: Dioecesis Alexandrina–Cornubiensis) was a Roman Catholic diocese that comprised the easternmost part of the Province of Ontario. The diocese was created by Pope Leo XIII on January 23, 1890. It has also previously operated under the name the Roman Catholic Diocese of Alexandria in Ontario. On May 6, 2020 the diocese was merged with the Archdiocese of Ottawa to form the Roman Catholic Archdiocese of Ottawa–Cornwall.
The see of Alexandria-Cornwall was vacant following the appointment of then-bishop Marcel Damphousse as the Bishop of Sault Sainte Marie on November 12, 2015. Archbishop Terrence Thomas Prendergast of the Archdiocese of Ottawa, was appointed Apostolic Administrator on January 13, 2016, and became Bishop of Alexandria-Cornwall on April 27, 2018, while remaining Archbishop of Ottawa. The two dioceses were united "in persona episcopi" ("in the person of the Bishop").On December 12, 2018, Pope Francis named Guy Desrochers as Titular Bishop of Melzi and Auxiliary Bishop of Alexandria-Cornwall (with episcopal ordination to take place in St. Finnan's Cathedral, Alexandria on February 22, 2019).
As of 2004, the diocese contained 32 parishes, 36 active diocesan priests, 9 religious priests, and 56,000 Catholics. It also had 32 Women Religious, 13 Religious Brothers, and 16 permanent deacons.
The following is a list of the bishops of Alexandria-Cornwall, and their terms of service: