The Irtysh () is a river in Russia, China, and Kazakhstan. It is the chief tributary of the Ob and is also the longest tributary in the world. The river's source lies in the Mongolian Altai in Dzungaria (the northern part of Xinjiang, China) close to the border with Mongolia. The Irtysh's main tributaries include the Tobol, Demyanka and the Ishim. The Ob-Irtysh system forms a major drainage basin in Asia, encompassing most of Western Siberia and the Altai Mountains.
