The Prussian Constitution of 1920 (German: Verfassung von Preußen 1920) formed the legal framework for the Free State of Prussia, a constituent state of the Weimar Republic, from 1918 to 1947. It was based on democratic parliamentary principles and replaced the Constitution of 1848/50. During Nazi Germany, it was eroded to the point of irrelevance and following World War II lost legal force when the state of Prussia was abolished by the Allies in 1947.

The Constitution provided for a Landtag (parliament) elected by proportional representation under universal suffrage for both men and women, and a State Council (Staatsrat) elected by local parliaments to represent the interests of Prussia's provinces. Executive authority rested with the State Ministry, headed by a minister president who was elected by and responsible to the Landtag. He appointed and directed the other ministers and was responsible for setting policy. Unlike the Weimar Constitution, the 1920 Prussian Constitution did not provide for a president. While it remained in force, Prussia was among the most stable and democratic states of the Weimar Republic.

Until the German Revolution of 1918–1919, Prussia was governed by the 1850 Constitution of the Kingdom of Prussia. It was an amended version of the Constitution of 1848 that King Frederick William IV had imposed following the German revolutions of 1848–1849. It established a constitutional monarchy with a two-chamber parliament and included a catalog of fundamental rights, but the king, who could veto any law, bypass the judiciary and had control of the military, remained the strongest power in the state. The Constitution's most controversial feature, which Social Democrats in particular criticized during the lifetime of the German Empire, was the three-class franchise that weighted votes based on the amount of taxes paid.

After the revolution, existing laws initially continued to apply except where they explicitly contradicted the new democratic order. The Prussian House of Representatives and House of Lords were therefore dissolved on 15 November 1918. The three-class franchise system was also replaced by universal suffrage for men and women. The Prussian State Assembly, which was to draw up a new constitution for Prussia, was elected under the expanded franchise on 26 January 1919.

The decision to elect an assembly to discuss a future democratic Prussian constitution was made at the meetings of the revolutionary Prussian Council of People's Deputies on 12 and 14 December 1918, when Otto Braun, Paul Hirsch and Eugen Ernst of the Majority Social Democratic Party (MSPD) succeeded in getting the more radical Independent Social Democratic Party (USPD) to abandon its opposition. The elections were to take place one week before the elections to the Weimar National Assembly, which was to adopt a national constitution for Germany. Hugo Preuß, who went on to write the draft version of the Weimar Constitution for the National Assembly, advocated splitting up the state of Prussia because of its size (almost two-thirds of both Germany's population and area) and therefore wanted to have the Prussian State Assembly meet only after fundamental decisions had been made at the national level. The early voting date was part of the interim Prussian government's attempt to prevent a breakup of the state.

The election to the State Assembly took place on 26 January 1919, one week after (rather than before) the elections for the Weimar National Assembly. It was the first statewide election in Prussia which was held under universal, equal and secret suffrage for men and women instead of the three-class system. Of 401 deputies elected, 26 were women. The voter turnout was about 74%.

The following parties gained enough votes to win at least one seat:

The Assembly convened for its constituent session on 13 March 1919. A total of 27 members were chosen for the constitutional committee that was responsible for drafting the constitution. Eleven were from the MSPD, six from the Centre Party, four the German Democratic Party (DDP), four the German National People's Party (DNVP), and one each from the USPD and the German People's Party (DVP).