| Native Name: | Grafschaft Holstein-Ploen |
| Conventional Long Name: | County of Holstein-Plön |
| Common Name: | Holstein-Plön |
| Era: | Middle Ages |
| Status: | imperial county |
| Empire: | Holy Roman Empire |
| Year Start: | 1290 |
| Year End: | 1390 |
| Event Start: | Partition of Holstein-Itzehoe following death of Gerhard I |
| Image Map Caption: | Holstein-Plön within Holstein-Kiel and neighbouring territories circa 1400 |
| Government Type: | monarchy |
| Title Leader: | Counts of Schauenburg |
| Capital: | Plön |
| House1: | House of Schauenburg |
| Religion: | Roman Catholic |
| Common Languages: | Northern Low Saxon, German |
| P1: | Holstein-Itzehoe |
| S1: | Holstein-Rendsburg |
| Flag S1: | Holstein Arms.svg |
| Event End: | Reversion in 1390 to Holstein-Rendsburg after the comital male line died out |
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.