Generalplan Ost (GPO) (English: Master Plan East) was a secret Nazi German plan for the colonization of Eastern Europe. Implementing it would have necessitated genocide and ethnic cleansing on a vast scale to be undertaken in the Eastern European territories occupied by Germany during World War II. It would have included the extermination of most Slavic people in Europe. The plan, prepared in the years 1939-1942, was part of Adolf Hitler's and the Nazi movement's Lebensraum policy and a fulfilment of the Drang nach Osten (English: Drive towards the East) ideology of German expansion to the east, both of them part of the larger plan to establish the New Order.

The body responsible for the drafting of this plan was the Reich Main Security Office (Reichssicherheitshauptamt - RSHA), the security organ of the SS responsible for fighting all enemies of Nazism. It was a strictly confidential document, and its contents were known only to those at the topmost level of the Nazi hierarchy.

According the testimony of SS-Standartenführer Dr. Hans Ehlich (one of the witnesses in Case VIII before the Subsequent Nuremberg Trials), the final version of the plan was drafted in 1940. As a high official in the RSHA, Ehlich was the man responsible for the drafting of Generalplan Ost. It had been preceded by the Ostforschung, a number of studies and research projects carried out over several years by various academic centres to provide the necessary facts and figures. The preliminary versions were discussed by the SS head Heinrich Himmler and his most trusted colleagues even before the outbreak of war. This was mentioned by SS-Obergruppenführer Erich von dem Bach-Zelewski during his evidence as a prosecution witness in the trial of officials of the SS-Rasse- und Siedlungshauptamt (RuSHA) (SS Office of Race and Settlement).