The Duchy of Schleswig-Holstein-Sonderburg-Plön, also Schleswig-Holstein-Plön, Holstein-Plön or just Duchy of Plön, was a small sub-duchy (Teilherzogtum) created by the physical division of the Duchy of Schleswig-Holstein-Sonderburg. Today, its remaining significance is primarily the building of Plön Castle. The Duchy of Plön was not a territorial dukedom in its own right, but a sub-division within the state structure of the duchies of Schleswig and Holstein. The scattered territorial dominion lay mostly in the southeast part of present-day German state of Schleswig-Holstein.

Schleswig-Holstein-Sonderburg was the name of a branch line of the House of Oldenburg as well as the name of their land. It existed from 1564 until 1668 and was a titular duchy under the King of Denmark, rather than a true territorial dukedom in its own right. The seat of the duke was Sønderborg. Parts of the domain were located in Denmark, mainly on the islands of Als and Ærø and around Glücksburg, whilst other lands were part of the Holy Roman Empire, including the Ämter of Plön, Ahrensbök, and Reinfeld. As a result of various inheritance arrangements it fragmented into numerous small territories which were eventually absorbed into Greater Denmark in the 18th century.