Kvarnsjön is a lake in Stockholm County, Södermanland, Sweden.

Kvarnsjön
View of lake
Location of lake
Location of lake
Kvarnsjön
Coordinates59°11′2″N 17°58′18″E / 59.18389°N 17.97167°E / 59.18389; 17.97167
Basin countriesSweden

It is part of the Tyresån lake system.