The Roman Catholic Diocese of Trebinje-Mrkan is an ecclesiastical territory or diocese of the Roman Catholic Church in eastern Herzegovina.
It is the oldest and smallest Roman Catholic diocese of Bosnia and Herzegovina. The diocese was formed sometime before the year 1022. An exact date is not known, but church officials decided to celebrate the diocese's millennium in 1984, estimatiing its foundation at 984. The diocese has its episcopal see in the city of Trebinje. However, from 1361 to 1391 it was relocated to the island of Mrkan, thus taking on the present double name.
In 1890, the diocese was united with the much larger Diocese of Mostar-Duvno. Since then it has been administered by Mostar-Duvno's bishop (currently Ratko Perić). However, unlike the rest of the Mostar-Duvno territory, Franciscan priests are not appointed to parishes in Trebinje-Mrkan, as the Franciscan custody does not extend into this region.
On May 1, 1984 the Church of the Birth of Mary in Trebinje was made a cathedral by decree. This came to coincide with a symposium held that year under the leadership of Bishop Pavao Žanić to study the historical and pastoral significance of the diocese. One of the most important parishes in the bishopric is the Shrine of the Queen of Peace in Hrasno. Every year thousands gather to celebrate the parish's feast day.