Koper, having arisen from an ancient settlement built on an island in the southeastern part of the Gulf of Koper in the northern Adriatic, was known as Aegida (Goat Town) in Ancient Greek, later becoming known by the Latin names of Capris (Greek Copros), Caprea, Capre, or Caprista, from which the modern Slovenian name stems.

Roman citizens of nearby Tergeste (modern Trieste) had fled to Capris in 568 due to an invasion of the Lombards.

The town, renamed Justinopolis in honor of the emperor Justinian II, was later under both Lombard and Frankish rule and was briefly occupied by Avars in the eighth century.

Koper has been the seat of a diocese since the eighth century, possibly even since the sixth century.

Trade between Koper and Venice has been recorded since 932.

Koper is on the German side in the war between Venice and the Holy Roman Empire, and as a result is granted town rights by Emperor Conrad II in 1035.