The China–Mongolia border is the international border between China and Mongolia. It extends 4,630 kilometers (2,880 miles) from the western tripoint with Russia in the Altai Mountains to the eastern tripoint near the Greater Khingan mountains. It is the world's fourth longest international border.^[1]

The border runs from west to east, beginning at the western tripoint with Russia in the Altai Mountains, just east of the China–Kazakhstan–Russia tripoint. From there, it stretches southeastward, predominantly through the Gobi Desert, with several straight-line sections, reaching Mongolia’s southernmost point north of the 40°30’ line of latitude. The boundary then curves northeast, crossing the Greater Khingan mountains and the Khalkhyn Gol river, before passing through Buir Lake and reaching the eastern tripoint with Russia.^[2]

During the 17th century, Russia expanded into Siberia, bringing it into conflict with Qing China, which then ruled over Outer Mongolia. The border as it stands today was influenced by the Treaty of Kyakhta, which established much of the Mongolia–Russia border.^[3][4]

Much of the boundary traverses arid and semi-arid regions, including the vast Gobi Desert. Notable natural landmarks along the border include the Altai Mountains in the west and Buir Lake in the east.^[5][6]

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