Herzberg Castle (German: Schloss Herzberg) is a German schloss in Herzberg am Harz in the district of Osterode am Harz in the state of Lower Saxony. The present-day, quadrangular building has its origins in the 11th century as a medieval castle. After a fire in 1510 it was rebuilt as a schloss and is one of the few in Lower Saxony that was constructed as a timber-framed building. Because it belonged to the House of Welf for 700 years it is also known as the Welf Castle of Herzberg (Welfenschloss Herzberg).
Herzberg Castle stands on a wooded eminence (275 m above sea level (NN)) immediately above and west of the centre of the town of Herzberg am Harz, which lies on the southwestern edge of the Harz Mountains. A stretch of the River Sieber runs past the castle to the north.
The present castle is an enclosed four-winged building with a rectangular courtyard (40 x 58 m) and was rebuilt after a serious fire in November 1510. Since the new castle was completed in 1528 its basements have been made of sandstone. One wing has an upper storey of stone, while the upper floors of the other three wings have been constructed using timber-framing. Its access through a gate tower and adjoining barbican has been retained. The old castle and the inner courtyard of the new one are entered after passing through the two-story gatehouse. The castle tower, known as the Clock Tower (Uhrturm), was built in the eastern corner. Its three upper storeys are made of timber.
Originally the schloss was a medieval castle that was based on a hunting lodge built on the spot between 1024 and 1029. The castle was probably built at the behest of King, later Emperor, Lothair III, known as Lothair of Süpplingenburg.