From its origins as the Kara-Irtysh (Black Irtysh) in the Mongolian Altay mountains in Xinjiang, China, the Irtysh flows northwest through Lake Zaysan in Kazakhstan, meeting the Ishim and Tobol rivers before merging with the Ob near Khanty-Mansiysk in western Siberia, Russia after .
The name Black Irtysh (Kara-Irtysh in Kazakh, or Cherny Irtysh in Russian) is applied by some authors, especially in Russia and Kazakhstan, to the upper course of the river, from its source entering Lake Zaysan.