Josias Rantzau (Bothkamp, near Kiel, 18 October 1609[1] – Paris, 14 September 1650[2]) was a Danish military leader and Marshal of France.

He married his Cousin Hedwig Margarethe Elisabeth vor Rantzau, but had no children.

[3] 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,[7] but is in general better remembered for his bravery than for his military skills.

On 30 June 1645, he became Marshal of France[9] and in 1646 Governor of the newly conquered Dunkirk Fortress.