The Act of Tilsit (Lithuanian: Tilžės aktas) was an act, signed in Tilsit by 24 members of the National Council of Lithuania Minor (Lithuanian: Mažosios Lietuvos tautinė taryba) on November 30, 1918.^[1] Signatories demanded unification of Lithuania Minor and Lithuania Proper into a single Lithuanian state. This would mean detaching the northern areas of East Prussia, inhabited by Prussian Lithuanians, from the German Empire.

The part of East Prussia north of Neman River, the Memel Territory up to the city of Memel (Klaipėda), was detached by Polish efforts by the Treaty of Versailles and placed under the supervision of the League of Nations. The rest of East Prussia, located south of the Neman River, including the town of Tilsit, where the act was signed, remained within Germany.

The Act was not signed by the main pro-Lithuanian oriented Prussian Lithuanian leaders Wilhelm Storost and Wilhelm Gaigalat. The latter was elected as chairman of the Prussian Lithuanian Council, but refused to take the position.^[2] He was replaced by general secretary Erdmonas Simonaitis. The act was widely published in the Republic of Lithuania only rather than in Lithuania Minor. It, among other facts, suggest the act primarily being a "show" document, intended to widely present an opinion of the National Council of Lithuania Minor.^[3]

Eventually, the Act of Tilsit became an important propaganda tool during the staged Klaipėda Revolt of 1923, after which Memel Territory (Klaipėda Region) was annexed by Lithuania.^[1] In March 1939, Lithuania was forced to cede Klaipėda Region to Nazi Germany. Some^[1] of the signatories of the Tilsit Act were later persecuted by the Nazis for treason, and Simonaitis was sent to a Nazi concentration camp but survived.