Dießen am Ammersee (Southern Bavarian: Diaßn am Ammasä) is a municipality in the district of Landsberg in Bavaria in Germany.

Situated in the Bavarian Alpine Foreland the town stretches from the shores of the Ammersee to the forested morainic hills of the Saale and Würm glaciation.

The town area consists of the market town of Dießen (with St. Georgen, Wengen, Ziegelstadel, Bischofsried and Seehof) and the former independent boroughs of Rieden (with Riederau, Bierdorf, Lachen, St. Alban and Romenthal), Dettenschwang (with Oberhausen, Unterhausen, Wolfgrub and Abtsried), Dettenhofen (with Pitzeshofen, Engenried, Hübschenried and Ummenhausen) and Obermühlhausen (with Oberbeuern, Unterbeuern and Schlöglhof).

In the course of the war between Ludwig the Bavarian and Frederick the Fair Dießen was burned in 1318 and 1320 by the troops of Duke Leopold.

The market Dießen itself was under the control of ducal, later electoral judges, who also held jurisdiction over the Ammersee and since 1599 also over the forested area west of the town called Forst Bayerdießen.

In the War of the Spanish Succession Dießen was again looted in 1703 by Austrian troops, the population fled again to Erlaich island in the lake.

The monastery church, the Marienmünster, was rebuilt in the years 1732 to 1739 by the Baroque master builder Johann Michael Fischer.

While the monastery shortly held the jurisdiction over the town of Dießen, by 1326 the free market won all its rights back.

The jurisdiction area of the Augustiner-Chorherrenstift itself included Raisting, Lachen, Bierdorf, Engenried, St. Alban, Riederau, Rieden, St. Georgen, Wengen, Bischofsried and Pitzeshofen.

From Easter Sunday to Kirchweih, the four passenger ships of the Bayerische Seenschifffahrt can be used to reach other places on the Ammersee.