The Roman Catholic Diocese of Alexandria–Cornwall 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 Archdiocese of Ottawa–Cornwall is a Catholic archdiocese that includes part of the Province of Ontario and includes the suffragan dioceses of Hearst, Pembroke, and Timmins. On 6 May 2020, Pope Francis amalgamated the Archdiocese of Ottawa and the Diocese of Alexandria-Cornwall to create the Archdiocese of Ottawa–Cornwall.