The Struma or Strymónas (Bulgarian Струма, , Greek Στρυμόνας [striˈmonas], Turkish Karasu 'black water') is a river in Bulgaria and Greece. Its ancient name was Strymōn (Greek: Στρυμών /stry'mɔːn/). Its catchment area is 10,800 km². It takes its source from the Vitosha Mountain in Bulgaria, runs first westward, then southward, enters Greek territory at the Kula village and flows into the Aegean Sea, near Amphipolis in the Serres prefecture. The river's length is 415 km (of which 290 km in Bulgaria, making it the country's fifth longest).
The river valley is a coal-producing area of Bulgaria. The Greek portion is a valley which is dominant in agriculture, being Greece's fourth biggest valley. The tributaries include the Rila River, the Dragovishtitsa, the Blagoevgradska Bistritsa, the Konska River, the Sandanska Bistritsa and the Aggitis River.
The Ancient Greek city of Amphipolis was founded near the river's entrance to the Aegean, at the site previously known as Ennea Odoi (Nine roads}. When Xerxes I of Persia crossed the river during his invasion in 480 BC he buried alive nine young boys and nine maidens as a sacrifice to the river god. The forces of Alexander I of Macedon defeated the remnants of Xerxes' army near Ennea Odoi in 479 BC. In 424 BC the Spartan general Brasidas after crossing the entire Greek peninsula sieged and conquered Amphipolis. The Battle of Kleidion was fought by the river in 1014.
In 1913, the Greek Army was trapped in the Kresna Gorge of the Struma during the Second Balkan War. The Bulgarians were defeated in the war, however, and the Treaty of Bucharest resulted in significant territorial losses for Bulgaria.
The river's name comes from Thracian Strymón, derived from IE *sru "stream" , akin to English stream, Old Irish sruaimm "river", Lithuanian straumuoe "fast stream", Greek reuma "stream", Albanian rrymë "water flow", shri "rain".