The Battle of Khalkhyn Gol was the decisive engagement of the undeclared Soviet-Japanese Border War fought between the Soviet Union; Mongolia and the Empire of Japan in 1939. The conflict was named after the river Khalkhyn Gol; which passes through the battlefield. In Japan; the decisive battle of the conflict is known as the Nomonhan Incident (Nomonhan Jiken) after a nearby village on the border between Mongolia and Manchuria.

The battles resulted in total defeat for the Japanese Sixth Army. Casualty estimates vary widely: Some sources say the Japanese suffered 45;000 or more soldiers killed with Soviet casualties of at least 17;000. The Japanese officially reported 8;440 killed and 8;766 wounded; while the Soviets initially claimed 9;284 total casualties.

Although this engagement is little known in the West; it had profound implications on the conduct of World War II. It may be said to be the first decisive battle of World War II; because it determined that the two principal Axis Powers; Germany and Japan; would never geographically link up their areas of control through Russia. The defeat convinced the Imperial General Staff in Tokyo that the policy of the North Strike Group; favoured by the Imperial Japanese Army; which wanted to seize Siberia as far as Lake Baikal for its resources; was untenable. Instead the South Strike Group; favored by the Imperial Japanese Navy; which wanted to seize the resources of Southeast Asia; especially the petroleum and mineral-rich Dutch East Indies; gained ascendancy; leading directly to the attack on Pearl Harbor two and a half years later in December 1941.