The Hexenturm is a stone tower in the municipality of Sarnen in the canton of Obwalden in Switzerland. It is a Swiss heritage site of national significance. The name refers to it being used as a prison for suspected witches in the 17th century. Today it houses the Cantonal Archives of Obwalden.
The tower was built around 1285/86 as the residence tower of larger castle complex, the Unteren Burg von Sarnen or Lower Sarnen Castle. The castle was built for the von Kellner family who were knights in service to the Murbach Abbey. The first member of the family to appear in records is the cellarius Heinrich at the monastery in 1229. The family name may be a form of the title and office that he held. The sons of Heinrich and his brothers were the knights Niklaus and Heinrich Kellner who built the castle. Niklaus probably lived in Sarnen, while his brother lived in Lucerne. In 1291 the Habsburgs bought the town of Lucerne and the Unterwalden estates, including the castle and surrounding farms, from Murbach Abbey. The Kellner family became vassals of the Habsburg-Laufenburg line. When the Everlasting League was created on 1 August 1291, the Kellners found themselves at odds with their neighbors and by 1308 they had been driven out. The last Kellner, Heinrich, died in 1348.