Amelungsborn Abbey, also Amelunxborn Abbey (Kloster Amelungsborn) near Negenborn and Stadtoldendorf, in the Landkreis of Holzminden in the Weserbergland, was the second oldest Cistercian foundation in Lower Saxony, Germany, after Walkenried Abbey. It survived the Reformation by becoming Lutheran, and with Loccum Abbey, also previously Cistercian, is one of the only two Lutheran monasteries in Germany with an uninterrupted tradition. The abbey church, St. Mary's, is also the parish church of the abbey's former estate villages Negenborn and Holenberg.
The site of the villa Amelungsborn to the west of the present Stadtoldendorf was originally part of the ancestral lands of the Counts of Northeim.
Siegfried IV, the last Count of Northeim-Boyneburg and Homburg gave the land at Amelungsborn for the foundation of a Cistercian monastery, which was officially settled by a community of monks from Altenkamp Abbey on 20 November 1135.
With the establishment of this monastery and of the nearby Burg Homburg, built at around the same time, it seems that Count Siegfried was aiming to secure a part of his possessions that lay distant from his ancestral seat in North Hessen.
No foundation charter has survived, although there is a confirmation dated 5 December 1129 by Pope Honorius II. Nevertheless an interval of six years between foundation and settlement fits the general timescale of Cistercian foundations.
The abbey was dedicated in 1135 by Bernhard I, Bishop of Hildesheim. The first abbot of Amelungsborn, appointed in 1141, was Heinrich I, a half-brother of the founder, Count Siegfried IV.