Neighboring districts are (from north clockwise) Westerwaldkreis, Limburg-Weilburg, Rheingau-Taunus, Mainz-Bingen, Rhein-Hunsrück, Mayen-Koblenz, and the district-free city Koblenz.

When Nassau lost independence in 1866 it was added to Prussia, who then in 1867 created the Regierungsbezirk Wiesbaden, and as parts of it the two districts Rheingaukreis and Unterlahnkreis.

The Rhine forms the boundary to the west, its narrow valley is used for wine cultivation.

The Lahn flows through the northern part of the district until it joins the Rhine near Lahnstein.

The two colors red and blue represent the two districts which were merged to form the Rhein-Lahn district - the blue derives from Nassau, the red from the clerical states of Trier and Mainz.