From Wikipedia, the free encyclopedia: Istria (/ˈɪstriə/ ISS-tree-ə; Croatian and Slovene: Istra; Istriot: Eîstria; Istro-Romanian, Italian and Venetian: Istria; formerly Histria in Latin and Ἵστρια in Ancient Greek) is the largest peninsula within the Adriatic Sea. The peninsula is located at the head of the Adriatic between the Gulf of Trieste and the Kvarner Gulf. It is shared by three countries: Croatia, Slovenia, and Italy. Croatia encapsulates most of the Istrian peninsula within Istria County.