Unterbacher See, Recreational lake in eastern Düsseldorf, Germany.

Unterbacher See is an 83.6-hectare gravel pit lake, up to 13.4 m deep, in the Unterbach district of south-east Düsseldorf. It has no inlets or outlets and is fed only by groundwater. Four islands in the water are designated bird protection areas, and the surrounding Unterbacher See/Eller Forst nature reserve is mostly open woodland with hiking and cycling paths.

Gravel and sand dredging between 1926 and 1973 created the lake. During the Second World War the German Luftwaffe laid out a decoy airfield here to mislead Allied bombers. On 7 February 1956 Düsseldorf, Erkrath and Hilden founded the Zweckverband Erholungsgebiet Unterbacher See, which still manages the site.

The lake's four bird-protection islands and the surrounding Unterbacher See/Eller Forst reserve shape how it is used: walking and cycling paths run through the woods, while the water itself hosts sailing, rowing and dragon-boat clubs, among them the Düsseldorfer Segelclub Unterbacher See and the Rheindrachen dragon-boat club. The southern shore also has a nudist beach.

Parking is available at both Strandbad Nord and Strandbad Süd, but fills up quickly in good weather; arriving by bus or bicycle avoids this. The two banks are linked by paths around the water, and the Elbsee on the far side of the A46 can be reached on foot or by bike through an underpass.

During the Second World War, a decoy airfield was laid out here to mislead Allied bombers. The lake itself was dug for gravel and sand between 1926 and 1973, and four islands in the water are now designated bird protection zones.