The Köpet Dag, Kopet Dagh, or Koppeh Dagh (; ), also known as the Turkmen-Khorasan Mountain Range, is a mountain range on the border between Turkmenistan and Iran that extends about along the border southeast of the Caspian Sea, stretching northwest-southeast from near the Caspian Sea in the northwest to the Harirud River in the southeast. The highest peak of the range in Turkmenistan is the Mount Rizeh (Kuh-e Rizeh), located at the southwest of the capital Ashgabat and stands at . The highest Iranian summit is Mount Quchan (Kuh-e Quchan) with .