| Conventional Long Name: | Principality of Leyen |
| Common Name: | Leyen |
| Era: | Napoleonic Wars |
| Status: | Client state |
| Status Text: | Client state of the French Empire Member of the Confederation of the Rhine |
| Empire: | First French Empire |
| Government Type: | Principality |
| Year Start: | 1806 |
| Year End: | 1814 |
| Event Start: | County of Adendorf raised to principality |
| Event End: | Mediatised to Austria by Congress of Vienna |
| Event Post: | Granted to Baden |
| Date Post: | 1819 |
| P1: | County of Adendorf |
| Flag P1: | Shield of Leyen.svg |
| Border P1: | no |
| S1: | Austrian Empire |
| Flag S1: | Flag of the Habsburg Monarchy.svg |
| Image Map Caption: | The Principality of Leyen, as shown within the Grand Duchy of Baden |
| Capital: | Hohengeroldseck |
| Leader1: | Philip Francis |
| Title Leader: | Prince |
The Principality of Leyen was a Napoleonic German state which existed 1806–1814 in Hohengeroldseck, in the west of modern Baden-Württemberg. The House of Leyen had acquired many districts in western Germany, and eventually these were inherited by the Leyen line of counts at Adendorf. 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'. The territory formed an enclave surrounded by Baden. 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. In 1819, Austria traded it to Baden.