Which country shares the longest border with Russia?

The country that shares the longest border with Russia is Kazakhstan. The Kazakhstan–Russia frontier stretches across northern Kazakhstan and southern Russia, forming the longest continuous international land border in the world.

The border runs for roughly 7,600 kilometers, from the Caspian Sea in the west to the tripoint where Kazakhstan, Russia, and China meet in the east. Much of it follows open steppe, making the boundary geographically different from Russia's more mountainous or forested borders elsewhere. The modern international border largely follows the former administrative boundary between the Kazakh and Russian Soviet republics.

China is another major Russian neighbor and is often guessed because its border is historically and strategically important, but it is substantially shorter. Ukraine and Finland also share notable borders with Russia, yet neither approaches Kazakhstan's length. Kazakhstan became independent in 1991 after the dissolution of the Soviet Union, turning the former internal boundary into an international frontier.