The main rivers of the district are the Lahn and the Dill, which also gave it the name. It is mountain landscape with the Westerwald in the west, the Taunus in the south, the Lahn-Dill-Bergland in the east and the Rothaargebirge of the Sauerland in the north. The highest elevation with 671 m is near Rittershausen (part of Dietzhölztal), the lowest with 135 is in the Lahn valley in Biskirchen (part of the town of Leun).
The coat of arms shows the imperial eagle in the top, a sign for the imperial city Wetzlar. The bottom part - taken from the arms of the previous Dillkreis - shows the lion of Nassau and the horn of the princes of Orange-Nassau, which later became the Dutch royal family. Dillenburg was the capital of the county of Nassau and later of Nassau-Orange.