Ordensburg (plural Ordensburgen) is a German term meaning a "castle of a (military) order". It is used specifically for the fortified structures built by crusading German military orders during the Middle Ages.

Ordensburgen were originally constructed by the Livonian Brothers of the Sword and later by the Teutonic Order to fortify territory in Prussia and Livonia captured from the native populations – Old Prussians, Lithuanians and native peoples of what is now Latvia and Estonia. Later, Ordensburgen were used to attack Lithuania.

Since they were built and used by religious military orders, the Ordensburgen often resembled cloisters. While they were considerably larger than those in the Holy Roman Empire, they were much scarcer in the Monastic state of the Teutonic Knights.

While a normal castle in the Reich would control about 38 km², a castle would control 370 km² in Prussia and 789 km² in Livonia, Courland and Estonia. The few small castles are considered to be of vassals, while the larger ones might have served as arsenals and strongholds during the Northern Crusades. They were purpose-built to colonize the respective countries and peoples by capturing (and holding) territory.

Most Ordensburgen were rectangular, even quadratic in form, built from red brick and lacking a Bergfried. Many castles had no towers at all, as the bailey (a mighty quadrangle) was considered sufficient for defence.

• NS-Ordensburgen
• List of castles in Estonia
• List of castles in Latvia
• List of castles in Lithuania
• List of castles in Poland

• Krahe, Friedrich-Wilhelm (2000). Burgen des deutschen Mittelalters. Grundriss-Lexikon (in German). Flechsig. ISBN 3-88189-360-1.