The Diocese of Mostar-Duvno is a Latin Church diocese of the Catholic Church in Bosnia and Herzegovina. It encompasses northern Herzegovina. The episcopal seat is in Mostar, Bosnia and Herzegovina. It was formed on 5 July 1881, when the Apostolic Vicariate of Herzegovina was elevated to the diocese. It is the largest Catholic diocese in Bosnia and Herzegovina by the number of Catholics.

Herzegovina is the southern and smaller of two main geographical regions of Bosnia and Herzegovina, the other being Bosnia. It has never had strictly defined geographical, cultural or historical borders, nor has it ever been defined as an administrative whole in the geopolitical and economic subdivision of Bosnia and Herzegovina.