The Dieksee is a lake in the Holstein Switzerland region of North Germany.

The lake is up to 38 metres deep, its surface elevation is about 22 m above sea level (NN) and it is crossed by the River Schwentine from east to west.

This waterway links the Dieksee to the other lakes in the Holstein Lake District (the Kleiner- and Großer Eutiner See, the Kellersee, the Großen- and Kleinen Plöner See and the Ukleisee).

Like the rest of Holstein Switzerland, the Dieksee was formed during the last ice age.

There is a circular walk and cycle path around lake which is about 14 kilometres long.