The Maritsa or Evros is, with a length of 480 km, the longest river that runs solely in the interior of the Balkans. It has its origin in the Rila Mountains in Western Bulgaria, flowing southeast between the Balkan and Rhodope Mountains, past Plovdiv and Parvomay to Edirne, Turkey. East of Svilengrad, Bulgaria, the river flows eastwards, forming the border between Bulgaria (on the north bank) and Greece (on the south bank), and then between Turkey and Greece. At Edirne, the river flows through Turkish territory on both banks, then turns towards the south and forms the border between Greece on the west bank and Turkey on the east bank to the Aegean Sea.

Turkey was given a small sector on the west bank opposite the city of Edirne. The river enters the Aegean Sea near Enez, where it forms a delta. The Tundzha is its chief tributary; the Arda is another one. The lower course of the Maritsa/Evros forms part of the Bulgarian-Greek border and most of the Greek-Turkish border. The upper Maritsa valley is a principal east-west route in Bulgaria. The unnavigable river is used for power production and irrigation.