The Generalplan Ost (German pronunciation: [ɡenəˈʁaːlˌplaːn ˈɔst]; lit. 'Master Plan for the East'), abbreviated GPO, was Nazi Germany's plan for the settlement and "Germanization" of captured territory in Eastern Europe, involving the genocide, extermination and large-scale ethnic cleansing of Slavs, Eastern European Jews, and other indigenous peoples of Eastern Europe categorized as "Untermenschen" in Nazi ideology. The campaign was a precursor to Nazi Germany's planned colonisation of Central and Eastern Europe by Germanic settlers, and it was carried out through systematic massacres, mass starvations, chattel labour, mass rapes, child abductions, and sexual slavery.

Generalplan Ost was only partially implemented during the war in territories occupied by Germany on the Eastern Front during World War II, resulting indirectly and directly in the deaths of millions by shootings, starvation, disease, extermination through labour, and genocide. However, its full implementation was not considered practicable during major military operations, and never materialised due to Germany's defeat. Under direct orders from Nazi leadership, around 11 million Slavs were killed in systematic violence and state terrorism carried out as part of the GPO. In addition to genocide, millions more were forced into slave labour to serve the German war economy.

The program's operational guidelines were based on the policy of Lebensraum proposed by Adolf Hitler and the Nazi Party in fulfilment of the Drang nach Osten (drive to the East) ideology of German expansionism. As such, it was intended to be a part of the New Order in Europe. Approximately 3.3 million Soviet POWs captured by the Wehrmacht were killed as part of the GPO. The plan intended for the genocide of the majority of Slavic inhabitants by various means – mass killings, forced starvations, slave labour and other occupation policies. The remaining populations were to be forcibly deported beyond the Urals, paving the way for German settlers.

The plan was a work in progress. There are four known versions of it, developed as time went on. After the invasion of Poland, the original blueprint for Generalplan Ost was discussed by the RKFDV in mid-1940 during the Nazi–Soviet population transfers. The second known version of the GPO was procured by the RSHA from Erhard Wetzel [de] in April 1942. The third version was officially dated June 1942. The final version of the Master Plan for the East came from the RKFDV on October 29, 1942. However, after the German defeat at Stalingrad, resources allocated to colonization policies were diverted to Axis war efforts, and the program was gradually abandoned. Had Generalplan Ost been fully implemented, it is estimated that more than 60 million people would have perished.

Generalplan Ost was Nazi Germany's plan for the colonization and Germanization of Central and Eastern Europe over a period of twenty-five years. 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 and de-population of most Slavic people in Eastern 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. More than economic calculations, ideological fanaticism and racism played a central role in Nazi regime's implementation of extermination programs such as the GPO. Hitler's doctrine of Lebensraum envisaged the mass-killings, enslavement and ethnic cleansing of Slavic inhabitants of Eastern Europe, followed by the colonization of these lands with Germanic settlers.

Although racist views against Slavs had precedent in German society before Hitler's rule, Nazi anti-Slavism was also based on the doctrines of scientific racism. The "Master Race" doctrine of Nazi ideology condemned Slavs to permanent domination by Germanic peoples, since it viewed them as primitive people who lacked the ability to undertake autonomous activities. Generalplan Ost evolved from these racist, imperialist ideas and was formulated by the Nazi regime as its official policy during the course of the Second World War.

"... when we speak of new territory in Europe today we must principally think of Russia and the border states subject to her. Destiny itself seems to wish to point out the way for us here. In delivering Russia over to Bolshevism, fate robbed the Russian people of that intellectual class which had once created the Russian state and were the guarantee of its existence. For the Russian state was not organized by the constructive political talent of the Slav element in Russia, but was much more a marvellous exemplification of the capacity for state-building possessed by the Germanic element in a race of inferior worth. ... This colossal empire in the East is ripe for dissolution. And the end of the Jewish domination in Russia will also be the end of Russia as a state. We are chosen by destiny to be the witnesses of a catastrophe which will afford the strongest confirmation of the nationalist theory of race."