Ossiach Abbey (Stift Ossiach) is a former Benedictine monastery in Ossiach near Feldkirchen in Kärnten in the Austrian state of Carinthia.
The monastery was established as the first in the medieval Duchy of Carinthia in about 1024 by a certain Count Ozi, affiliated with the Styrian Otakar dynasty and father of Patriarch Poppo of Aquileia. The first monks were probably from Niederaltaich Abbey in Bavaria. From the Otakars the Vogtei of Ossiach passed to the Austrian House of Babenberg and finally fell to the Habsburgs.
Ossiach Abbey was dissolved in 1783, after which the buildings were used as a barracks. In 1816 the premises were largely demolished. Between 1872 and 1915 the few remaining buildings were again used as a barracks and as stabling. Since 1995 the premises have been owned by the administration of Carinthia. The church since the dissolution has served the local parish. Two stained glass windows were donated by Karl May in 1905, though according to recent research the popular writer had probably never visited Ossiach.
The monastery site is one of the venues of the annual music festival "Carinthian Summer".