Keizersberg Abbey, also known as Mont César Abbey (Dutch: Abdij van Keizersberg; French: Abbaye du Mont-César) is a Benedictine monastery on the hill Keizersberg or Mont César in the north of the university town of Leuven, Belgium.

The Keizersberg ("Caesar's" or "Emperor's hill") was the site of the castle around which the city of Leuven grew up, and which local legend connected with Julius Caesar. The castle was demolished in 1782 by order of Emperor Joseph II. On the east side of the same hill a commandery of the Knights Templars was built in 1187, which when the order was abolished came to the Knights Hospitallers in 1312. This was secularised by the French in 1798, when the church and larger buildings were demolished.

A Benedictine house of studies was established in Leuven in 1888 by Dom Gerard van Caloen, rector of the abbey school of Maredsous Abbey, and land was acquired on the present site in the following year for the construction of a larger establishment, in which the remains of the old commandery were incorporated. The first major conventual block, the north wing, was completed in 1897. The abbey was formally founded on 13 April 1899 as part of the Beuron Congregation, under the first abbot, Dom Robertus de Kerchove.

Columba Marmion, was abbot of Maredsous and also appointed prior of Mont César in 1906, which he remained until his death in 1923. He was beatified in 2000.

The foundation is perhaps best known in the English-speaking world under its French name, Mont César Abbey, for its connection both with Blessed Columba and with the reformer and ecumenist Dom Lambert Beauduin, who while a member of this community launched a liturgical movement in 1909 and began publishing the accompanying periodical Les Questions Liturgiques et Paroissiales the following year. Dom Lambert left Mont César in 1925 to be prior of Amay Priory, established from Mont César. From there he later founded Chevetogne Abbey.

In 1914 the buildings were severely damaged by fire, and the monks took refuge in another Beuronese house, Maria Laach Abbey in the Rhineland, until after the end of the war.

In 1921 the abbey moved from the Beuron Congregation to become part of the new Belgian Congregation of Our Lady (Belgische Congregatie van Onze-Lieve-Vrouw-Boodschap), under the presidency of Abbot Robertus. In 1929 publication began of the theological journal "Recherches de Théologie Ancienne et Médiévale".

Dom Bruno (Henri Reynders), famous for hiding many Jewish children from the Nazis during World War II, was a monk of Mont César from 1922 until 1968. A fellow monk at the abbey provided him with several skillfully forged identification cards.

The abbey again suffered damage in World War II with the bombing of the buildings during air raids on Leuven in 1944, which among other things destroyed the last remains of the older buildings from the time of the Hospitallers, and the monastery was temporarily uninhabitable.

By 1948 it was sufficiently restored to be able to set up a small community at Wavreumont in Stavelot, which was formally established as St. Remaclus' Priory on 21 June 1952.

When the French section of the Catholic University of Leuven was moved out of Leuven it was decided that the abbey should become a Flemish institution, whence the change from Mont César to Keizersberg. On 10 June 1968 the abbey was transferred to the Flemish Province of the Subiaco Congregation. The abbot and prior resigned in the same year, and a temporary administrator was appointed.

In 1969 part of the renovated abbey was converted for use as student accommodation, and is still used for that purpose.

In 2020, a part of the Medieval wall collapsed.

As of 2022, there were five monks in residence at Keizersberg.

• Dom Robertus de Kerchove 1899-1928
• Dom Bernard Capelle 1928-1952
• Dom Rombout Van Doren 1952-1968

Dom Filips De Cloedt (acting abbot-administrator) 1968-1970

• Dom Ambroos Verheul 1970-1991

Dom Livien Bauwens (acting prior-administrator) 1991-1993

• Dom Kris Op de Beeck 1993-2017

Dom Dirk Hanssens was appointed as prior-administrator in 2018

• Keizersberg Abbey website (in Dutch)
• Benedictines - Abbey Keizersberg, Leuven in ODIS - Online Database for Intermediary Structures Archived 2016-04-28 at the Wayback Machine
• Archives of Benedictines - Abbey Keizersberg, Leuven in ODIS - Online Database for Intermediary Structures Archived 2016-04-28 at the Wayback Machine

50°53′19″N 4°41′48″E / 50.8885°N 4.6966°E / 50.8885; 4.6966