Mariental Abbey (German: Kloster Mariental) in the present Mariental in Lower Saxony, Germany, is a former Cistercian monastery founded in 1138, along with Marienberg Abbey in Helmstedt, by Pfalzgraf Friedrich II von Sommerschenburg.
It was a daughter house of Altenberg Abbey of the filiation of Morimond. The initial complement consisted of twelve monks from Altenberg under an abbot (Bodo) from Amelungsborn Abbey. In 1179 the von Sommerschenburgs died out and the monastery with its possessions fell into the hands of Henry the Lion.
The abbey became extremely wealthy: its estates extended as far as Magdeburg, Jüterbog and Braunschweig. In 1232 it established a daughter house of its own, Hude Abbey. However, at the end of the 14th century it entered upon a steady decline.
The monastery was dissolved in 1569 as a consequence of the Protestant Reformation. A Lutheran school had been set up in the premises in 1542, and this lasted until 1745. A seminary for the training of teachers was also established here, and this continued until 1773, when it was transferred to Helmstedt.
The buildings today belong to the Braunschweigische Vereinigte Kloster- und Studienfonds, while the church has become the Protestant parish church of Mariental.