The Diocese of Koper is a Latin diocese of the Catholic Church in southwestern Slovenia. It is part of the ecclesiastical province of Ljubljana. Its cathedral is dedicated to the Assumption of Mary and is located in the Adriatic port town of Koper. A co-cathedral, the Co-Cathedral of Christ the Saviour, located in Nova Gorica, gained its status in 2004. The Latin name of the diocese, Dioecesis Iustinopolitana, is due to the fact that Koper was in the past name Justinopolis in honour of the Byzantine emperor Justinian II.