The Diocese of Koper is a Roman Catholic diocese located in Slovenia. Established in the 6th century, it serves as a spiritual center for the local Catholic community. The diocese covers the southwestern part of Slovenia, including the coastal region along the Adriatic Sea. The diocese is part of the Metropolitan Archdiocese of Ljubljana and is led by a bishop. Its main church is the Cathedral of the Assumption in Koper, which is an important historical and architectural site. The diocese plays a significant role in promoting religious activities and community services in the region.