Ostsiedlung is the term for the High Medieval migration period of ethnic Germans into the territories at the eastern part of the Holy Roman Empire that Germans conquered before and beyond; and the consequences for settlement development and social structures in the immigration areas. Generally sparsely and only recently populated by Slavic, Baltic and Finnic peoples, the area of colonization, also known as Germania Slavica, encompassed Germany east of the Saale and Elbe rivers, part of states of Lower Austria and Styria in Austria, the Baltics, Poland, the Czech Republic, Slovakia, Slovenia, Hungary, and Transylvania in Romania.

The majority of settlers moved individually, in independent efforts, in multiple stages and on different routes as there existed no imperial colonization policy, central planning or movement organization. Many settlers were encouraged and invited by the Slavic princes and regional lords.

Groups of migrants first moved to the east during the early Middle Ages. Larger treks of settlers, which included scholars, monks, missionaries, craftsmen and artisans, often invited, in numbers unverifiable, first moved eastwards during the mid 12th century. The military territorial conquests and punitive expeditions of the Ottonian and Salian emperors during the 11th and 12th centuries are not attributable to the Ostsiedlung, as these actions didn't result in any noteworthy settlement establishment east of the Elbe and Saale rivers. The Ostsiedlung is considered to have been a purely Medieval event as it ended in the beginning of the 14th century. The legal, cultural, linguistic, religious and economic changes caused by the movement had a profound influence on the history of Eastern Central Europe between the Baltic Sea and the Carpathians until the 20th century.