Heimburg Castle (German: Burg Heimburg), also called the Altenburg or Alteburg, is a ruined castle on an oval hilltop about 330 metres above sea level (NN) which is located just north of the Harz Mountains in central Germany.

The ruins of this hilltop castle stand above the village of Heimburg in the borough of Blankenburg in the district of Harz in the state of Saxony-Anhalt.

In 1143 the castle must have been rebuilt as it was entrusted to a ministerialis, Anno of Heimburg, by Henry the Lion.

In the 16th century the castle was enfeoffed repeatedly but after its destruction in the Thirty Years' War it was not restored.

After the Regensteins died out in 1599 the fief was returned to its feudal lord, the Duke of Brunswick.