Koper (Slovene pronunciation: [ˈkoːpəɾ] (listen); Italian: Capodistria, Croatian: Kopar) is the fifth largest city in Slovenia. Located in the Istrian region in the southwestern part of the country, approximately five kilometres (3.1 miles) south of the border with Italy and 20 kilometres (12 miles) from Trieste, Koper is the largest coastal city in the country. It is bordered by the satellite towns of Izola and Ankaran. With a unique ecology and biodiversity, it is considered an important natural resource. The city's Port of Koper is Slovenia's only container port and a major contributor to the economy of the Municipality of Koper. The influence of the Port of Koper on tourism was one of the factors in Ankaran deciding to leave the municipality in a referendum in 2011 to establish its own municipality. The city is a destination for a number of Mediterranean cruising lines. Koper is the main urban centre of the Slovenian Istria, with a population of about 25,000. Aleš Bržan is the current mayor, serving since 2018.

The city of Koper is officially bilingual, with both Slovene and Italian as its official languages. Sights in Koper include the 15th-century Praetorian Palace and Loggia in Venetian Gothic style, the 12th-century Carmine Rotunda church, and St. Nazarius' Cathedral, with its 14th-century tower.

Koper is also one of the main road entry points into Slovenia from Italy, which lies to the north of the municipality. The main motorway crossing is at Spodnje Škofije to the north of the city of Koper. The motorway continues into Rabuiese and Trieste. Koper also has a rail connection with the capital city, Ljubljana. On the coast, there is a crossing at Lazaret into Lazzaretto in Muggia municipality in Trieste province. The Italian border crossing is known as San Bartolomeo.