Flachsee is an artificial lake on the Reuss at Rottenschwil, south of Bremgarten in the Aargau region of Switzerland. The reservoir was formed after the construction of the dam in 1975 at Bremgarten-Zufikon. The lake's surface area is 72 ha (180 acres).

• List of lakes of Switzerland

• "Flachsee" (PDF). Archived from the original (PDF) on 2013-10-19. Retrieved 2013-10-18. (828 KiB) (in German) description of lake and nature reserve