The Struma (Bulgarian: Струма, Greek: Strimón, Turkish: Karasu (meaning "black water" in Turkish)) is a river in Bulgaria and Greece. Its ancient name was Strymon.

The Struma rises in the Vitosha Mountains of Bulgaria and flows about 400 kilometers south to the Aegean Sea. The valley is a coal-producing area of Bulgaria.

The ancient Greek city of Amphipolis was founded at the river's entrance to the Aegean, and the Battle of Kleidion was fought by the river in 1014.