The Free State of Prussia was one of the constituent states of Germany from 1918 to 1947. The successor to the Kingdom of Prussia after the defeat of the German Empire in World War I, it continued to be the dominant state in Germany during the Weimar Republic, as it had been during the empire, even though most of Germany's post-war territorial losses in Europe had come from its lands. It was home to the federal capital Berlin and had 62% of Germany's territory and 61% of its population. Prussia changed from the authoritarian state it had been in the past and became a parliamentary democracy under its 1920 constitution. During the Weimar period it was governed almost entirely by pro-democratic parties and proved more politically stable than the Republic itself. With only brief interruptions, the Social Democratic Party (SPD) provided the Minister President. Its Ministers of the Interior, also from the SPD, pushed republican reform of the administration and police, with the result that Prussia was considered a bulwark of democracy within the Weimar Republic.

The Kingdom of Prussia constituted the German state of Prussia between 1701 and 1918. It was the driving force behind the unification of Germany in 1866 and was the leading state of the German Empire until its dissolution in 1918. Although it took its name from the region called Prussia, it was based in the Margraviate of Brandenburg. Its capital was Berlin.