The Battles of Khalkhin Gol were the decisive engagements of the undeclared Soviet–Japanese border conflicts involving the Soviet Union, Mongolia, Japan and Manchukuo in 1939. The conflict was named after the river Khalkhin Gol, which passes through the battlefield. In Japan, the decisive battle of the conflict is known as the Nomonhan Incident after Nomonhan, a nearby village on the border between Mongolia and Manchuria. The battles resulted in the defeat of the Japanese Sixth Army.

The Soviet–Japanese border conflicts, also known as the Soviet-Japanese Border War, the First Soviet-Japanese War, the Russo-Mongolian-Japanese Border Wars or the Soviet-Mongolian-Japanese Border Wars, were a series of minor and major conflicts fought between the Soviet Union, Mongolia and Japan in Northeast Asia from 1932 to 1939.