Ossiach Abbey (Stift Ossiach) is a former Benedictine monastery in Ossiach, in the Austrian state of Carinthia. The site is one of the venues of an annual music festival called "Carinthian Summer".
In 878 the East Frankish king Carloman of Bavaria dedicated the Treffen estates around Lake Ossiach to the Benedictine monastery of Ötting. In the late 10th century the lands passed to the Bishops of Passau and later to Emperor Henry II, who conferred them to a certain Count Ozi, affiliated with the Styrian Otakar dynasty and father of Patriarch Poppo of Aquileia.
A church probably already existed at Ossiach, when Count Ozi about 1024 established the Benedictine abbey, the first in the medieval Duchy of Carinthia. The first monks probably descended from Niederaltaich Abbey in Bavaria. Ozi's son Poppo succeeded in removing the proprietary monastery from the influence of the Salzburg archbishops and to affiliate it with the Patriarchate of Aquileia, confirmed by Emperor Conrad II in 1028. Upon the extinction of the Styrian Otakars in 1192, the Vogtei of Ossiach according to the Georgenberg Pact passed to the Austrian House of Babenberg. In 1282 it finally fell to the Habsburgs.