In 1973, as part of that year's reform of Baden-Württemberg's administrative structure [de], the district of Überlingen was merged into a new one based at Sigmaringen.

[2] The municipality (Gemeinde) of Illmensee is situated at the southeastern corner of the district of Sigmaringen, in the German state of Baden-Württemberg.

The municipality is physically located in a depression formed by the Würm glaciation in the Upper Swabian hill country [de].

[2] The Federally-protected Ruschweiler and Volzer Lakes [de] nature reserve is located in Illmensee's municipal area.

[2] Illmensee's coat of arms depicts a white fish leaping over a yellow, three-pointed hill upon a field of red.