The Sumbar (also Sari-su, Sara-su and Ṣáríṣú) is a fast flowing river in southern Turkmenistan and northern Iran. It a tributary of the Atrek. The name Sari-su means yellow water in Turkic languages, but is applied to a number of other rivers as well. The Sumbar is 245 km long and drains a basin of 8300 km². It arises in the Kopet Dag mountains in Iran and flows into Turkmenistan. For a long stretch before the Sumbar runs into the Atrek, it is separated from it by a range of hills called the Marábeh. The Atrek becomes the Turkmenistan-Iran border where the Sumbar flows into it.