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.

The territory included Vilnius, the historical capital of the Grand Duchy of Lithuania. Lithuania, after declaring independence from the Russian Empire, claimed the Vilnius Region based on this historical legacy. Poland argued for the right of self-determination of the local Polish-speaking population. As a result, throughout the interwar period the control over the area was disputed between Poland and Lithuania. The Soviet Union recognized it as part of Lithuania in the Soviet–Lithuanian Peace Treaty, but in 1920 it was seized by Poland and became part of the short-lived puppet state of Central Lithuania, and was subsequently incorporated into the Second Polish Republic.

Direct military conflicts (Polish–Lithuanian War and Żeligowski's Mutiny) were followed up by fruitless negotiations in the League of Nations. After the Soviet invasion of Poland in 1939, as part of the Soviet fulfilment of the Molotov–Ribbentrop Pact, the entire region was occupied by the Soviet Union. About one-fifth of the region, including Vilnius, was ceded to Lithuania by the Soviet Union on 10 October 1939 in exchange for Soviet military bases within the territory of Lithuania as part of the Soviet–Lithuanian Mutual Assistance Treaty. The remaining part of the region was given to the Byelorussian Soviet Socialist Republic.

The conflict over Vilnius Region was settled after World War II when both Poland and Lithuania were in the Eastern Bloc, as Poland was the Soviet satellite state of the Polish People's Republic and Lithuania was occupied by the Soviet Union as the Lithuanian Soviet Socialist Republic, and Poles were repatriated to Poland. From the late 1940s to 1990, the region was divided between the Lithuanian SSR and Byelorussian SSR, and since 1990 between modern-day independent Lithuania and Belarus.

Initially, the Vilnius Region did not possess exact borders per se, but encompassed Vilnius and the surrounding areas. This territory was disputed between Lithuania and Poland after both countries had successfully reestablished their independence in 1918. Later, the western limit of the region became a de facto administration line between Poland and Lithuania following Polish military action in autumn 1920. Lithuania refused to recognize this action or the border. The eastern limit was defined by the Soviet–Lithuanian Peace Treaty. The eastern line was never turned into an actual border between states and remained only a political vision. The total territory covered about 32,250 km² (12,450 sq mi).

Today the eastern limit of the region lies between the Lithuanian and Belarusian border. This border divides the Vilnius Region into two parts: western and eastern. The Western Vilnius Region, including Vilnius, is now part of Lithuania. It constitutes about one-third of the total Vilnius Region. Lithuania gained about 6,880 km² (2,660 sq mi) on October 10, 1939, from the Soviet Union and 2,650 km² (1,020 sq mi) (including Druskininkai and Švenčionys) on August 3, 1940, from the Byelorussian SSR. The Eastern Vilnius Region became part of Belarus. No parts of the region are in modern Poland. None of the countries have any further territorial claims.

The term Central Lithuania refers to the short-lived puppet state of the Republic of Central Lithuania, proclaimed by Lucjan Żeligowski after his staged mutiny in the annexed areas. After eighteen months of existing under Poland's military protection, it was annexed by Poland on 24 March 1922 thus finalizing Poland's claims over the territory.

In the Middle Ages, Vilnius and its environs had become a nucleus of the early ethnic Lithuanian state, the Duchy of Lithuania, also referred to in Lithuanian historiography as a part of the Lithuania Propria, that became Kingdom of Lithuania and later Grand Duchy of Lithuania.