Nienburg Abbey, Benedictine abbey in Nienburg (Saale), Germany

Nienburg Abbey is a former Benedictine monastery in Nienburg an der Saale, Saxony-Anhalt. Its abbey church of St Mary and St Cyprian, dedicated in 1004, is an early Gothic hall church with Romanesque elements, built mainly in the 13th century and counted among the earliest Gothic hall churches in central Germany. The conventual buildings were rebuilt between 1680 and 1690 as a castle.

The Benedictine community founded in 970 at Thankmarsfelde in the Harz was moved to the fortress of Nienburg in 975 to convert the region's Sorbian population; the rebuilding took over 30 years. Emperor Otto II made the abbey a Reichskloster, and it grew into one of the wealthiest monasteries of the region, holding scattered estates such as Harzgerode, Niemitsch and Lübben. In 1166 Frederick Barbarossa transferred it to the Archbishops of Magdeburg, ending its independence.

Around 1150 the imperial chronicle of the Annalista Saxo was written at Nienburg Abbey. The abbey church of St Mary and St Cyprian, dedicated in 1004, belongs today to the Evangelische Landeskirche Anhalts and is used ecumenically. Gero II, Margrave of the Saxon Ostmark, and Thietmar, Margrave of Meissen, are buried here.

The abbey church of St Mary and St Cyprian, dedicated in 1004, is still in use and belongs to the Evangelische Landeskirche Anhalts; it is used ecumenically. It is a station on the Straße der Romanik, the tourist route through Romanesque monuments of Saxony-Anhalt. The conventual buildings converted into a castle stand next to it; parts were destroyed by a fire in 1996.

In 1996 a fire started by children playing destroyed parts of the former abbey and castle buildings. The site had been sold in 1871 to an industrialist who converted it into a malt factory. The conventual buildings themselves were rebuilt between 1680 and 1690 as a widow's residence for the Princes of Anhalt.