Rhein-Lahn is a district (Kreis) in the east of Rhineland-Palatinate, Germany. Neighboring districts are (from north clockwise) Westerwaldkreis, Limburg-Weilburg, Rheingau-Taunus, Mainz-Bingen, Rhein-Hunsrück, Mayen-Koblenz, and the district-free city Koblenz.
With the Congress of Vienna the area was added to the duchy of Nassau. When Nassau lost indepence 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 Rheingaukreis became the district St. Goarshausen in 1885. In 1969 the two districts were merged into the new Rhein-Lahn district.
The name of the district already mentions the two biggest rivers of the district. 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. In the southern part of the district are the hills of the Taunus.