Mattsee is a market town at the eponymous lake in the district of Salzburg-Umgebung in the Austrian state of Salzburg.

In ca. 765, Duke Tassilo III of Bavaria established the Mattsee Benedictine Abbey, which 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, the college has existed since 1807. The abbey church, a Gothic building, has a Baroque equipment and a prominent 60m/197 ft high steeple added in 1766. Adjacent is a museum showing paintings of Johann Michael Rottmayr, an astronomical clock, and an 860 grant deed by King Louis the German.

The lake's water has a reputation of being of excellent quality, being fed by underground springs.

The fahr(T)raum museum exhibits the car collection of Ernst Piëch, grandson of Ferdinand Porsche.

• Anton Diabelli, composer, born in Mattsee on September 6, 1781, died on April 8, 1858, in Vienna
• Dorothea Seyss-Inquart, daughter of Arthur Seyss-Inquart (born 7 May 1928, still alive as of 2008, living in Mattsee, Upper Austria ).

• Bühl (Germany, since 1972)