Opatija is a town and a municipality in Primorje-Gorski Kotar County in northwestern Croatia. The traditional seaside resort on the Kvarner G…Opatija is a town and a municipality in Primorje-Gorski Kotar County in northwestern Croatia. The traditional seaside resort on the Kvarner Gulf is known for its Mediterranean climate and its historic buildings reminiscent of the Austrian Riviera.