| Native Name: | Reichsgrafschaft Rantzau |
| Conventional Long Name: | Imperial County of Rantzau |
| Common Name: | Rantzau |
| Status: | Imperial county |
| Empire: | Holy Roman Empire |
| Year Start: | 1650 |
| Year End: | 1864 |
| Event1: | Acquired by Denmark-Norway |
| Date Event1: | 9 April 1726 |
| Event End: | Mediatised to Prussia |
| P1: | Holstein-Gottorp |
| S1: | Kingdom of Prussia |
| Flag S1: | Flag of the Kingdom of Prussia (1803-1892).svg |
| Capital: | Barmstedt |
The Imperial County of Rantzau (German: Reichsgrafschaft Rantzau) was an immediate state of the Holy Roman Empire which lasted from 1650 to 1864. Its territory is more or less congruent with the present Amt Rantzau.
In 1649, Frederick III, Duke of Holstein-Gottorp, sold his part of the Lordship of Pinneberg, which had formerly belonged to the County of Schauenburg, to (1614–1663), royal Danish governor of Holstein. In 1650 or 1651, Rantzau became an immediate county and state of the Holy Roman Empire. In 1726, it was annexed by the Danish rulers, after (1688-1734), had murdered his brothers and was imprisoned. Wilhelm Adolf died in 1734 and Rantzau was inherited by the Duchy of Holstein, which was reigned by the Danish kings and its secundogenitures.