The Caspian Sea (also known as Mazandaran Sea, Hyrcanian Ocean, or Khazar Sea) is the world's largest inland body of water, variously classed as the world's largest lake or a full-fledged sea. An endorheic basin, it lies between Europe and Asia; east of the Caucasus, west of the broad steppe of Central Asia, south of the fertile plains of Southern Russia in Eastern Europe, and north of the mountainous Iranian Plateau of Western Asia. It covers 371,000 km2 (143,000 sq mi) (excluding the highly saline lagoon of Garabogazköl to its east) and a volume of 78,200 km3 (19,000 cu mi). It has a salinity of approximately 1.2% (12 g/l), about a third that of average seawater. It is bounded by Kazakhstan from mid-north to mid-east, Russia from mid-north to mid-west, Azerbaijan to the southwest, Iran to the south and adjacent corners, and Turkmenistan along southern parts of its eastern coast.

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The Caspian Sea is home to a wide range of species and may be best known for its caviar and oil industries. Pollution from the oil industry and, to a lesser extent, dams on rivers draining into it have harmed its ecology.