Lahnstein is a verband-free town of Rhein-Lahn-Kreis in Rhineland-Palatinate, Germany. It is situated at the confluence of the Lahn with the Rhine, approximately 6 kilometres (3.7 mi) south of Koblenz. Lahnstein was created in 1969 by the merger of the previously independent towns of Oberlahnstein on the south side of the Lahn and Niederlahnstein on the north side. In 2020, it had a population of 18,030.

The Lahn is a 245.6-kilometre-long (152.6 mi), right tributary of the Rhine in Germany. Its course passes through the federal states of North Rhine-Westphalia (23.0 km), Hesse (165.6 km), and Rhineland-Palatinate (57.0 km).