Nestos (Greek: Νέστος; Bulgarian: Mesta) is a river in Bulgaria and Greece. It rises in the Rila Mountains and flows into the Aegean Sea near the island Thasos. The length of the river is 230 km. It forms some gorges in Rila and Pirin. Its longest tributary is the Dospat River. The banks of the river are covered mainly by deciduous trees that extend into halfway between Bugaria and Greece where it forms the boundary between the Kavala and the Xanthi prefectures as well as Macedonia and Thrace. After the Treaty of Bucharest and prior to World War I it was also the border between Greece and Bulgaria.