The Schleswig-Holstein-Sonderburg-Franzhagen line was a short-lived name of the main line of the ducal house of Schleswig-Holstein-Sonderburg, after its bankruptcy in 1667. The name is derived from the Franzhagen Castle in Schulendorf in the duchy of Saxe-Lauenburg. The castle was inherited by Eleonore Charlotte of Saxe-Lauenburg-Franzhagen, who brought it into her marriage with former Duke Christian Adolph I. After their bankruptcy, the Sonderburg line was reduced to its possession of Franzhagen. The Franzhagen castle was demolished in 1716.
| Reign | Name | Remarks |
|---|---|---|
| 1676–1702 | Christian Adolf I | Former Duke of Schleswig-Holstein-Sonderburg, was declared bankrupt in 1667, the former headquarters Sonderburg Castle went to the Danish crown. In 1676, he married Eleonore Charlotte of Saxe-Lauenburg-Franzhagen, who brought eponymous castle Franzhagen into her marriage. |
| 1702–1707 | Christian Leopold | Son, married Anna Sophia Segelke morganatically, therefore, his children could not inherit his real estate |
| 1707–1708 | Charles Louis | Brother, married Anna Barbara Dorothea of Winterfeld morganatically. His children were ennobled, allowing them to inherit the castle. |
| 1708–1709 | Christian Adolf II | Underage son and heir. With him, the line died out. |
The real power of government from 1702 was wielded by Eleonore Charlotte of Saxe-Lauenburg-Franzhagen widow of Christian Adolf I, as the sons had married beneath her station. Anna Barbara Dorothea of Winterfeld owned until her death in 1739 only one house in Billwerder.