Flachsee

{{Short description|Lake on the Reuss at Rottenschwil, Switzerland}}{{Infobox lake

| name = Flachsee

| image = Flachsee Switzerland.jpg

| caption =

| image_bathymetry =

| caption_bathymetry =

| location = Aargau

| coords = {{coord|47|19|41|N|8|22|10|E|type:waterbody_region:CH-AG|display=inline,title}}

| type = reservoir

| inflow = Reuss

| outflow = Reuss

| catchment =

| basin_countries = Switzerland

| length =

| width =

| area = {{convert|72|ha|abbr=on}}

| depth =

| max-depth = {{convert|7|m|abbr=on}}

| volume =

| residence_time =

| shore =

| elevation = {{convert|378|m|abbr=on}}

| islands =

| cities =

| pushpin_map=Canton of Aargau#Switzerland#Alps

| pushpin_label_position= bottom

| embedded = {{Infobox mapframe |wikidata=yes |zoom=14 |coord={{WikidataCoord|display=i}}}}

}}

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 {{convert|72|ha|abbr=on}}.