Michaelstein Abbey (Kloster Michaelstein) is a former Cistercian monastery, now the home of the Stiftung Kloster Michaelstein - Musikinstitut für Aufführungspraxis ("Michaelstein Abbey Foundation - Music Institute for Performance"), near the town of Blankenburg in the Harz in Sachsen-Anhalt, Germany.
In a deed of Emperor Otto I dated 956 giving property to Quedlinburg Abbey is mentioned the cave church dedicated to Saint Michael, also known as the Volkmarskeller (as it is still called) near the Eggeröder spring. The same deed also mentions the cell of the revered anchorite Liutbirg, which traditionally was held to have been sited in or near the cave church. The holiness of the site proved attractive, and a religious community formed round it.
In 1139 Beatrix II, abbess of Quedlinburg, founded a Cistercian monastery here, which was settled in 1146 by monks from Kamp Abbey. A few years later the new monastery was transferred away from the cave church to form Michaelstein Abbey on the present site. The abbey's growing property brought it great wealth, but it never settled any daughter houses of its own.
The monastery was sacked in 1525 by rebellious mobs during the German Peasants' War. The church was ruined beyond repair, and was never rebuilt; religious services were held from then on in the former chapter house. In 1533 the remaining buildings were ransacked and devastated by Wilhelm von Haugwitz. In 1543 the last Roman Catholic abbot resigned, and the abbey and its assets passed into the hands of the Counts of Blankenburg, who acted as abbots. Under their rule the now Protestant community began a school in 1544.