The Hyrcanian forests are a zone of lush lowland and montane forests covering about 55,000 square kilometres (21,000 sq mi) near the shores of the Caspian Sea in Iran and Azerbaijan. The forest is named after the ancient region of Hyrcania. The World Wide Fund for Nature refers to the ecoregion as the Caspian Hyrcanian mixed forests. Since 5 July 2019, the Hyrcanian Forests have been designated a UNESCO World Heritage Site. In September 2023, the heritage site expanded to incorporate portions of the forest located in Azerbaijan.

The Caspian Sea is the world's largest inland body of water, often described as the world's largest lake and sometimes referred to as 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. It covers a surface area of 371,000 km2 (143,000 sq mi), an area approximately equal to that of Japan, with a volume of 78,200 km3 (19,000 cu mi). It has a salinity of approximately 1.2%, about a third of the salinity of average seawater. It is bounded by Kazakhstan to the northeast, Russia to the northwest, Azerbaijan to the southwest, Iran to the south, and Turkmenistan to the southeast.