Lahnau is a municipality in the Lahn-Dill-Kreis in Hesse, Germany, and lies about midway – about 6 km each way – between the towns of Wetzlar and Gießen.

(The population figures are drawn from a statistical extrapolation from 30 June 2003) The community of Dorlar had itself incorporated into Wetzlar in 1972.

Wetzlar, Gießen, Atzbach, Waldgirmes and 12 other municipalities formed from 1977 to 1979 the city of Lahn.

Within Lahn, the constituent communities of Waldgirmes, Dorlar and Atzbach the Stadtbezirk (roughly "borough") of Lahntal.

After the city of Lahn was dissolved, the borough became an independent community under the name Lahnau on 1 August 1979 The constituent community of Dorlar thereby managed to belong to four different municipalities over only seven years, which stands as a curiosity even in the complex history of Hessian municipal reform.