The statement was signed by the head of the Istrian prefect's office, Oriano Otocan, and the head of Friuli Venezia Giulia's department for international relations, Franco Lacop.
Otocan said that more than 300 European regions had their offices in Brussels, and that the northern Adriatic peninsula Istria was the first Croatian region to open one there.
County prefect Ivan Jakovcic said the opening of the office was very important for Istrian business people, companies and citizens, adding it also represented Istria's contribution to Croatia's European Union integration.
Jakovcic announced the office would be inaugurated in mid-March.