Burg Hachen, Medieval castle ruins in Sundern, Germany.

Burg Hachen is the ruin of a hilltop castle on a height above Hachen, part of Sundern in the Hochsauerlandkreis. Excavations in 1923 and 1978–1980 uncovered a main castle area of 40 by 60 metres, with an outer bailey on a plateau below. The endangered north side was protected by a 2-metre-thick shield wall, behind which stood a round tower 8 metres in diameter, later replaced by a trapezoidal building with thick walls.

Burg Hachen was built around the year 1000 and first belonged to Count Ezzo. In 1060 Otto of Northeim received it through his wife Richenza, and in 1080 his son Kuno of Beichlingen gave it to the Archbishop of Cologne, who enfeoffed the Counts of Dassel. They sold it in 1232 to the Counts of Arnsberg; after the county was sold in 1368 it fell back to the Cologne archbishopric. Around 1371 it served as widow's seat of Countess Anna of Arnsberg, who later preferred Wildshausen. From 1839 the community of Hachen owned the ruin.

The name Hachen comes from the settlement below the hill; the castle was always named after it, not after a noble family. Since 2019 the ruin and hill form the 2.5-hectare Burgberg Hachen nature reserve, protected for its snail fauna. Events are still allowed on the ruin if they do not conflict with the reserve's purpose.

The site can be visited freely and is accessible from the grounds. Sturdy shoes are recommended since the ground can be uneven and some walls may appear unstable.

The castle's build history is poorly documented; only a few charters from the 15th century survive. The Westphalian conservation office investigated the site in 1923 and again from 1978 to 1980, uncovering a main castle area of 40 by 60 metres. Behind a 2-metre-thick shield wall stood a round tower, possibly the keep, 8 metres across, later replaced by a trapezoidal building.