The Dragonja is a 30-kilometre (19 mi) long river in the northern part of the Istrian peninsula. It is a meandering river with a very branched basin and a small quantity of water. It has a pluvial regime and often dries up in summer. It features very diverse living environments and is home to a number of animal and plant species. The Dragonja has been a matter of a territorial dispute between Croatia and Slovenia, with its lowest portion de facto the border of the two countries.

Following the breakup of Yugoslavia in 1991, Slovenia and Croatia became independent countries. As the border between the countries had not been determined in detail prior to independence, several parts of the border were disputed, both on land and at the sea, namely in the Gulf of Piran.