The Soviet–Lithuanian Mutual Assistance Treaty was a bilateral treaty signed between the Soviet Union and Lithuania on October 10, 1939. According to provisions outlined in the treaty, Lithuania would acquire about one fifth of the Vilnius Region, including Lithuania's historical capital, Vilnius, and in exchange would allow five Soviet military bases with 20,000 troops to be established across Lithuania. In essence the treaty with Lithuania was very similar to the treaties that the Soviet Union signed with Estonia on September 28, and with Latvia on October 5. According to official Soviet sources, the Soviet military was strengthening the defenses of a weak nation against possible attacks by Nazi Germany. The treaty provided that Lithuania's sovereignty would not be affected. However, in reality the treaty opened the door for the first Soviet occupation of Lithuania and was described by The New York Times as "virtual sacrifice of independence."

Vilnius Region[a] is the territory in present-day Lithuania and Belarus that was originally inhabited by ethnic Baltic tribes and was a part of Lithuania proper, but came under East Slavic and Polish cultural influences over time.