Ostsiedlung (German: Settlement in the East), also known as German eastward expansion, refers to the medieval eastward migration and settlement of Germans from modern day Western and Central Germany into less-populated regions of Eastern Central Europe and Eastern Europe.

The affected area roughly stretches from Slovenia to Estonia.

In part, Ostsiedlung follows the territorial expansion of the Holy Roman Empire and the Teutonic Order.Preceding and along with German settlement, late medieval Central and Eastern Europe societies undergo deep cultural changes in demography, religion, law and administration, agriculture, settlement numbers and structures.

Thus, Ostsiedlung is part of a process termed Ostkolonisation (East colonization) or Hochmittelalterlicher Landesausbau (Late medieval rural development), though these terms are also used synonymously.