Klosterneuburg Abbey (German: Stift Klosterneuburg) is a twelfth-century Augustinian monastery of the Catholic Church located in the town of Klosterneuburg in Lower Austria. Overlooking the Danube, just north of the present Vienna city limits at the Leopoldsberg, the monastery was founded in 1114 by Saint Leopold III of Babenberg, the patron saint of Austria, and his second wife Agnes of Germany.

The abbey church, dedicated the Nativity of Mary (Mariä Geburt), was consecrated in 1136 and remodeled in the Baroque style in the seventeenth century. The impressive monastery complex was mostly constructed between 1730 and 1834. Its foundations, including a castle tower and a Gothic chapel, date back to the twelfth century. Other older buildings still extant within the complex include the chapel of 1318 with Saint Leopold's tomb. From 1634 on, the Habsburg rulers had the facilities rebuilt in the Baroque style, continued by the architects Jakob Prandtauer and Donato Felice d'Allio. The plans to embellish the monastery on the scale of an Austrian Escorial were later resumed by the Neoclassical architect Joseph Kornhäusel, though only small parts were actually carried out. In 1879, the abbey church and monastery were restored according to plans by Friedrich von Schmidt, and the neo-Gothic twin steeples were erected.

Klosterneuburg Monastery contains the Verduner Altar, made in 1181 by Nicholas of Verdun. Its three parts comprise 45 gilded copper plates modeled on Byzantine paragons, similar to the Shrine of the Three Kings at Cologne Cathedral. The monastery also contains a museum with a collection of Gothic and Baroque sculptures and a gallery of paintings, including fifteen-panel paintings by Rueland Frueauf from 1505, four Passion paintings from the backside of the Verduner Altar from 1331, and the Babenberg genealogical tree.

During the Investiture Controversy in the early twelfth century, Margrave Leopold III of Babenberg sided with the Papacy against Emperor Henry IV. In 1106, the emperor's son, Emperor Henry V, who sided with the Papacy against his father, rewarded Leopold's loyalty by offering him his sister Agnes' hand in marriage, in recognition of his services. Agnes was the widow of Duke Frederick I of Swabia. Leopold, who was recently widowed from his first wife, accepted the hand of this daughter of the Imperial Salian dynasty.

With this new connection to two imperial families, Leopold's status was elevated by the marriage, which also brought with it a large dowry of royal possessions. Following his marriage, Leopold initiated plans to build a castle on Leopoldsberg for his new residence, which had formerly been at Gars am Kamp and Tulln an der Donau. He named the new castle Niwenburc. According to legend, Leopold was standing with his wife on the balcony of their new castle when Agnes' veil was carried away by a strong gust of wind. The area was searched, but the veil was not found. Years later, Leopold was out hunting when he became attracted by a brilliant radiance coming from the foliage of an elderbush. The source of the light was the undamaged veil, entangled in the foliage. From the light emerged a vision of the Virgin Mary, who directed Leopold to build a church and monastery in her honour at that location.

In 1113, Leopold founded a monastery (kloster) for secular canons next to his castle, providing it with generous donations of land. The cornerstone ceremony for the new abbey church took place on 12 June 1114. Leopold's younger son, the chronicler Otto of Freising, prepared for his ecclesiastical career at Klosterneuburg and became provost in 1126. In 1133, Leopold handed the monastery over to the Augustinians after repossessing it from the secular canons. Leopold sought to create an impressive but private monastery next to his residence.

On 29 September 1136, the abbey church was consecrated after 22 years of construction. The form of that original basilica has survived for nine centuries, despite many subsequent modifications and reconstructions. Most likely the two side aisles had lofts, the middle aisle was most likely higher, and above the crossing there was a tower. Two months after the consecration, Margrave Leopold III died on 15 November 1136. Agnes survived him by seven years. In 1156, Duke Henry II of Austria moved his residence from Klosterneuburg to Vienna after receiving the ducal title. Despite the change, the monastery continued to develop as a religious and cultural institution.

In 1220, Duke Leopold VI of Austria selected a Burgundian master architect to build the Capella Speziosa chapel beside the convent. This chapel, considered among the most beautiful sacred buildings of its time, was demolished in the eighteenth century. On 13 September 1330, the town and monastery were seriously damaged in a fire. The monastery and abbey church were renovated, and new works of art were commissioned by provost Stephan of Sierndorf. In 1394, construction began on the south tower of the early-Gothic abbey church. It would take two centuries before the tower was completed in 1592.