Josias Rantzau (Bothkamp, near Kiel, 18 October 1609 – Paris, 14 September 1650) was a Danish military leader and Marshal of France.
Josias was the grandson of Paul, the youngest son of Johann Rantzau.
He married his Cousin Hedwig Margarethe Elisabeth vor Rantzau, but had no children.
As a young man, he served Prince Maurice of Orange and King Christian IV of Denmark.
In 1633, he successfully defended Andernach against the Spanish, but is in general better remembered for his bravery than for his military skills.
He was taken prisoner by the Spanish in 1642 at the Battle of Honnecourt, and again in 1643 by Imperial troops at the Battle of Tuttlingen.
On 30 June 1645, he became Marshal of France and in 1646 Governor of the newly conquered Dunkirk Fortress.
During the Fronde, he was arrested by Mazarin and locked up in the Bastille.
He was acquitted, but died shortly after being released.