The East Prussian plebiscite (German: Abstimmung in Ostpreußen), also known as the Allenstein and Marienwerder plebiscite or Warmia, Masuria and Powiśle plebiscite (Polish: Plebiscyt na Warmii, Mazurach i Powiślu), was a plebiscite for the self-determination of the regions of southern Warmia (Ermland), Masuria (Mazury, Masuren) and Powiśle, which had been in parts of the East Prussian Government Region of Allenstein and of the West Prussian Government Region of Marienwerder in accordance with Articles 94 to 97 of the Treaty of Versailles.

Prepared in early 1920, the plebiscite took place on 11 July 1920 and was conducted by German authorities, which were formally under Inter-Allied control.According to Richard K. Debo, both the German and the Polish governments believed that the outcome of the plebiscite had been decided by the ongoing Polish-Bolshevik War, which threatened the very existence of the newly-formed Polish state itself and so many Poles in the region voted for Germany for fear that if the area joined Poland, it would soon fall under Soviet rule. During the plebiscite, the Red Army came closer to Warsaw every day and committed crimes against the civilian population.

According to several Polish sources, the Germans engaged in a massive persecution of Polish activists and their Masurian supporters and went as far as engaging in regular hunts and murder to influence the vote. The organisation of the plebiscite was also influenced by Britain, which then supported Germany out of fear of an increased power for France in postwar Europe.According to Jerzy Minakowski, terror and their unequal status made Poles boycott the preparations for the plebiscite, which allowed the Germans to add ineligible voters.The German-conducted plebiscite reported that most voters had selected East Prussia over Poland (over 97% in the Allenstein Plebiscite Area and 92% in the Marienwerder Plebiscite Area). Most of the territories in question remained in the Free State of Prussia, and therefore, in Germany.