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.
In 1803 it was dissolved during the secularisation of Germany and fell into ruin. Starting in 1847 under King Frederick William IV of Prussia, a thorough restoration was carried out, and the restored church, known as the Altenberger Dom ("Altenberg Cathedral", although Altenberg was never the seat of a bishopric so this is not technically accurate), is now an interdenominational church used by both Roman Catholics and Protestants.