Holstein-Plön was a county (Grafschaft) in the Holy Roman Empire centred on Plön in Holstein. It was held by the House of Schauenburg who had ruled in Holstein and Stormarn since 1110/11. The county emerged before 1295 when the County of Holstein-Itzehoe was partitioned after the death of Count Gerhard I of Holstein-Itzehoe (died 1290) into the counties of Holstein-Plön, Holstein-Pinneberg and Holstein-Rendsburg.
The following counts ruled over Holstein-Plön:
Holstein-Itzehoe was a county that was formed from Schauenburg and Holstein by the division of Holstein between Gerhard and John in 1261.
Gerhard of Holstein-Itzehoe was the only regent. After his death in 1290 Holstein-Itzehoe was divided up between Gerhard's sons into the three counties of Holstein-Rendsburg, Holstein-Plön and Holstein-Pinneberg (the latter ruled in personal union with the County of Schauenburg, thus sometimes called Holstein and Schauenburg, or Holstein-Schauenburg, however Schauenburg was the name of the ruling dynasty in all the then Holsatian counties).
The House of Schaumburg was a dynasty of German rulers. Until c. 1485, it was also known as the House of Schauenburg. Together with its ancestral possession, the County of Schaumburg, the family also ruled the County of Holstein and its partitions Holstein-Itzehoe, Holstein-Kiel, Holstein-Pinneberg (till 1640), Holstein-Plön, Holstein-Segeberg and Holstein-Rendsburg (till 1460) and through the latter at times also the Duchy of Schleswig.