Rheinau Abbey (Kloster Rheinau) was a Benedictine monastery in Rheinau in the Canton of Zürich, Switzerland, founded in about 778 and suppressed in 1862. It is located on an island in the Rhine.
The foundation of the abbey, on a strategically sheltered bend of the Rhine, is supposed to have taken place in about 778. The abbey is first documented however in the 11th century. In 1114 a Romanesque basilica was dedicated here and in 1120 the still extant archive begun. The early history of the abbey, like that of many others, consists of an alternation between generous endowments and privileges from the Holy Roman Emperors, and oppression and fraud from the "Vögte" (lords protector). In 1126 Count Rudolf of Lenzburg founded the adjoining settlement of Rheinau.
Against the increasingly aggressive territorial claims of the Counts of Sulz the abbey made a treaty in 1455 with the Old Swiss Confederacy, which was intended to protect it against further attacks by the noble families of the Klettgau. In 1529 the Reformation swept in from Zürich and overwhelmed the abbey, which was abandoned shortly afterwards. It was re-established however in 1532, and became a centre of the Counter-reformation.
In the 18th century under Abbot Gerold II Zurlauben, Rheinau Abbey, like St. Gallen, enjoyed a late resurgence. Gerold had the abbey church (re-dedicated 1710) and the monastic complex (in construction up to 1744) magnificently re-built in the Baroque style, much as they appear today.