Mattsee is a market town in the district of Salzburg-Umgebung in the state of Salzburg in Austria. About 765 Duke Tassilo III of Bavaria established the Mattsee Benedictine Abbey, that became a part of the Diocese of Passau in 993 and was transformed into a college of canons. The Bishops of Passau had the Mattsee Castle built in the 12th century.
Under the jurisdiction of the Archdiocese of Salzburg since 1807, the college still exists today. The abbey church, a Gothic building, has a rich Baroque equipment and a prominent 60m/197ft high steeple added in 1766. Adjacent is a museum showing paintings of Johann Michael Rottmayr, an astronomical clock and a 860 grant deed by King Louis the German.