Ernest Casimir (22 December 1573[1] – 2 June 1632) was a Count of Nassau-Dietz and Stadtholder of Friesland, Groningen and Drenthe.

He was the 11th child of John VI, Count of Nassau-Dillenburg, and Countess Elisabeth of Leuchtenberg.

After the death of his father, his counties Nassau-Dillenburg, Nassau-Siegen, Nassau-Dietz, and Vianden were divided among his five living sons.

[2] Ernest Casimir was primarily known as an outstanding military leader during the Eighty Years' War.

[5] His son, Henry Casimir I, succeeded him as count of Nassau-Dietz and as Stadtholder of Friesland, Groningen and Drenthe.