Altenberg Abbey (Abtei Altenberg) (Latin: Vetus Mons) is a former Cistercian monastery in Altenberg, now a part of the municipality of Odenthal in the Bergisches Land, North Rhine-Westphalia, Germany.

The abbey 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.

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 currently an interdenominational church used by both Roman Catholics and Protestants.

Media related to Kloster Altenberg at Wikimedia Commons