Holstein-Plön was the name of a county ruled by the House of Schauenburg that ruled in Holstein and Stormarn from 1110/11 in the 12th century. 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: Following the death of Gerhard II his sons, Gerhard IV of Holstein-Plön and his younger half-brother John III, ruled jointly over Holstein-Plön. In 1314 Gerhard IV sold his lands to John III, who thus became the sole Count of Holstein-Plön. John III conquered Holstein-Kiel with Gerhard III of Holstein-Rendsburg. Holstein-Plön reverted to Holstein-Kiel in 1350 upon the death of 27-year-old Gerhard V without issue. When the Plön line died out in 1390, Holstein-Kiel (and Holstein-Plön within it) reverted to Holstein-Rendsburg, ruled by Count Nicholas of Holstein-Rendsburg (died 1397) and his nephew, Count Gerhard VI of Holstein-Rendsburg (died 1404). A circa-1400 map of the region shows the borders of Holstein-Rendsburg and its subordinate counties, Holstein-Kiel and Holstein-Plön.

Rise and fall of the county