The Principality of Leyen was a Napoleonic German state which existed 1806–1814 in Hohengeroldseck, in the west of modern Baden-Württemberg.

In 1797, France defeated the Holy Roman Empire and all lands west of the Rhine were lost.

Following the defeat of Austria in December 1805, most of the smaller German princely states were mediatized, with the glaring exception of Leyen, which was spared because the ruling Count was nephew to Archchancellor Karl Theodor von Dalberg,[1] a close collaborator of Napoleon's.

In 1806, Count Philip Francis of Adendorf was raised to a Prince, and his lands were renamed to the 'Principality of Leyen'.

Prince Philip Francis, like many other members of the Confederation of the Rhine became largely a French puppet, so following Napoleon's defeat at the Battle of Leipzig in 1813, the Congress of Vienna opted to mediatize his realm and give it to Austria.