Altenberg Abbey (Abtei Altenberg) (Latin: Vetus Mons) is a former Cistercian monastery in Altenberg, now a part of Odenthal in the Bergisches Land, North Rhine-Westphalia, Germany. It was founded in 1133 as a daughter house of Morimond Abbey and settled initially in the old castle of the Counts of Berg, Burg Berge, which the counts had left for Schloss Burg, but moved to the new purpose-built monastery in the valley of the Dhünn in 1153. It flourished sufficiently to undertake the settlement of a number of daughter houses of its own: Mariental Abbey and Wagrowiec Abbey, both in 1143; Lad Abbey in 1146; Zinna Abbey in 1171; Haina Abbey in 1188; Jüterbog Abbey in 1282; and Derneburg Abbey in 1443.