The Free City of Danzig (German: Freie Stadt Danzig; Polish: Wolne Miasto Gdańsk) was a city-state under the protection and oversight of the League of Nations between 1920 and 1939, consisting of the Baltic Sea port of Danzig (now Gdańsk, Poland) and nearly 200 other small localities in the surrounding areas. The polity was established on November 15, 1920, per the terms of Article 100 (Section XI of Part III) of the 1919 Treaty of Versailles, following the end of World War I.

Although predominantly German-populated, the territory was bound by the imposed union with Poland, which covered foreign policy, defense, customs, railways, and post, and remained distinct from both the post-war Weimar Republic and the newly independent Polish Republic. Additionally, Poland was granted certain rights related to port facilities in the city.

In the 1920 Constituent Assembly election, the Polish Party received over 6% of the vote; however, its percentage of votes later declined to approximately 3%. A large number of Danzig Poles voted for the Catholic Centre Party instead. In 1921, Poland began developing the city of Gdynia, a mid-sized fishing town. This new port, located north of Danzig, was established on territory awarded in 1919, known as the Polish Corridor. By 1933, the commerce passing through Gdynia exceeded that of Danzig. By 1936, the city's Senate had a majority of local Nazis, and agitation to rejoin Germany was stepped up. Many Jews fled from German persecution.

After the German invasion of Poland in 1939, the Nazis abolished the Free City and incorporated the area into the newly formed Reichsgau of Danzig-West Prussia. The Nazis classified the Poles and Jews living in the city as subhumans, subjecting them to discrimination, forced labor, and extermination at Nazi concentration camps, including nearby Stutthof (now Sztutowo, Poland). Upon the city's capture in the early months of 1945 by the Soviet and Polish troops, a significant number of German inhabitants perished in ill-prepared and over-delayed attempts to evacuate by sea, while the remainder fled or were expelled. The city was fully integrated into Poland due to the Potsdam Agreement, while members of the pre-war Polish ethnic minority began returning, and new Polish settlers started arriving. Gdańsk suffered severe underpopulation due to these events and did not recover until the late 1950s.

Danzig had an early history of independence. It was a leading player in the Prussian Confederation, which was directed against the Teutonic Monastic State of Prussia. The Confederation stipulated with the Polish king, Casimir IV Jagiellon, that the Polish Crown would be invested as head of state of the western parts of Prussia (Royal Prussia). In contrast, Ducal Prussia remained a Polish fief. Danzig and other cities, such as Elbing and Thorn, financed most of the warfare and enjoyed high city autonomy.

In 1569, when Royal Prussia's estates agreed to incorporate the region into the Polish–Lithuanian Commonwealth, the city insisted on preserving its special status. It defended itself through the Siege of Danzig in 1577 to protect its special privileges. Subsequently, it insisted on negotiating by sending emissaries directly to the Polish king. Danzig's location as a deep-water port where the Vistula River met the Baltic Sea had made it into one of the wealthiest cities in Europe in the 16th and 17th centuries, as grain from Poland and Ukraine was shipped down the Vistula on barges to be loaded onto ships in Danzig, where it was shipped on to western Europe. As many merchants shipping the grain from Danzig were Dutch and built Dutch-style houses for themselves, leading to other Danzigers imitating them, the city was thus given a distinctively Dutch appearance. Danzig became known as "the Amsterdam of the East", a wealthy seaport and trading crossroads that linked Western and Eastern European economies. Its location, where the Vistula flowed into the Baltic, led to various powers competing to rule the city.

Although Danzig became part of the Kingdom of Prussia following the Second Partition of Poland in 1793, Prussia was subsequently conquered by Napoleon Bonaparte in 1806. In September 1807, Napoleon declared Danzig a semi-independent client state of the French Empire, known as the Free City of Danzig. It lasted seven years, until it was reincorporated into the Kingdom of Prussia in 1814, following Napoleon's defeat at the Battle of Leipzig (also known as the Battle of Nations) at the hands of a coalition that included Russia, Austria, and Prussia.

Point 13 of U.S. President Woodrow Wilson's Fourteen Points called for Polish independence to be restored and for Poland to have "secure access to the sea", a promise that implied that Danzig, which occupied a strategic location where the Vistula River flowed into the Baltic Sea, should become part of Poland. At the Paris Peace Conference in 1919, the Polish delegation, led by Roman Dmowski, asked for Wilson to honor point 13 of the Fourteen Points by transferring Danzig to Poland, arguing that Poland would not be economically viable without Danzig and that since the city had been part of Poland until 1793, it was rightfully part of Poland anyway. However, Wilson had promised that national self-determination would be the basis of the Treaty of Versailles. As 90% of the people in Danzig in this period were German, the Allied leaders at the Paris Peace Conference compromised by creating the Free City of Danzig, a city-state in which Poland had certain special rights. It was felt that including a city that was 90% German into Poland would be a violation of the principle of national self-determination, but at the same time, the promise in the Fourteen Points of allowing Poland "secure access to the sea" gave Poland a claim on Danzig, hence the compromise of the Free City of Danzig.