The Kopet Dag, Kopet Dagh, or Koppeh Dagh (, ), also known as the Turkmen-Khorasan Mountain Range is a mountain range on the frontier 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 southwest of the capital Ashgabat and stands at . The highest Iranian summit is Mount Quchan (Kuh-e Quchan) with .