Vansjø is part of Moss watershed (Mossevassdraget) stretching from Østmarka. Vansjø has four inflow rivers. The lake is fed by the Hobølelva which drains lake Sværsvann, Veidalselva which originates in Hobøl as well as Svinndalselva and Mørkelva which originate in Våler. 
The lake has an area of 36.9 km² and an estimated coastline of about 250 km. Vansjø supplies drinking water in many parts of Moss region.