Maritsa or Maritza, also known as Meriç and Evros, is a river that runs through the Balkans in Southeast Europe. With a length of 480 km (300 mi), it is the longest river that runs solely in the interior of the Balkan peninsula, and one of the largest in Europe by discharge. It flows through Bulgaria in its upper and middle reaches, while its lower course forms much of the border between Greece and Turkey. Its drainage area is about 53,000 km2 (20,000 sq mi), of which 66.2% is in Bulgaria, 27.5% in Turkey and 6.3% in Greece. It is the main river of the historical region of Thrace, most of which lies in its drainage basin.

Thrace is a geographical and historical region in Southeast Europe. Bounded by the Balkan Mountains to the north, the Aegean Sea to the south, and the Black Sea to the east, it comprises present-day southeastern Bulgaria, northeastern Greece, and the European part of Turkey, roughly the Roman Province of Thrace. Lands also inhabited by ancient Thracians extended in the north to modern-day Northern Bulgaria and Romania and to the west into Macedonia.