Altenburg Abbey (Stift Altenburg) is a Benedictine monastery in Altenburg in Lower Austria.
It was founded in 1144 by Countess Hildeburg of Boigen-Rebgau. Under the abbots Maurus Boxler and Placidus Much the abbey took on its present Baroque form. Under the supervision of the architect Josef Munggenast some of the most distinguished artists and craftsmen of Austria worked on this project: Paul Troger on the frescoes, Franz Josef Holzinger on the stucco work, and Johann Georg Hoppl on the marbling.
Under the Emperor Joseph II in 1793 the abbey was forbidden to accept new novices, but unlike many others in Austria it succeeded in remaining in operation.
In 1940 under the National Socialists the abbey was suspended, and in 1941 dissolved: the abbot was placed under arrest and the community dispossessed. From 1945 the premises were used as accommodation by Soviet occupying troops. Under Abbot Maurus Knappek (1947–1968) the buildings were restored and the community renewed.
Since 1625 it has been a member of the Austrian Congregation now within the Benedictine Confederation.