The major Siberian river that forms a long river system with the Irtysh is the Ob.
The Ob begins at the confluence of the Biya and Katun rivers in Russia’s Altai region. Its chief tributary, the Irtysh, rises much farther south in China’s Mongolian Altai, crosses Kazakhstan, and joins the Ob near Khanty-Mansiysk in western Siberia. Together, the Ob and Irtysh form a river system about 5,410 km long.
After receiving the Irtysh, the Ob continues north through the West Siberian Plain and enters the Gulf of Ob, a vast estuary connected to the Kara Sea. The combined basin covers much of western Siberia and includes steppe, taiga, wetlands and Arctic tundra.
The Irtysh is longer than the Ob when measured from its own distant source, which can make the wording seem counterintuitive. Hydrologically, however, the Irtysh is the Ob’s principal tributary, so Ob is the canonical answer to this question.