The Diocese of Koper (Latin: Dioecesis Iustinopolitana; Slovene: Škofija Koper; Italian: Diocesi di Capodistria) 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.

The Latin name of the diocese, Dioecesis Iustinopolitana, is because Koper was in the past named Justinopolis in honour of the Byzantine emperor Justinian II.

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