Ernest Casimir I, Count of Nassau-Dietz

Ernst Casimir I of Nassau-Dietz (22 December 1573 in Dillenburg – 2 June 1632 in Roermond) was count of Nassau-Dietz and Stadtholder of Friesland, Groningen and Drenthe.

Sophia Hedwig and some children, by Paulus Moreelse.

Prince Frederick Henry and Count Ernst Casimir during the Siege of 's-Hertogenbosch

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

After the death of his father, the county of Nassau was divided among his five living sons, Ernst Casimir followed him as Count of Nassau-Dietz.

Ernst Casimir was primarily known as an outstanding military leader during the Eighty Years War. He served under Maurice of Nassau, Prince of Orange in the siege of the cities of Steenwijk and Oldenzaal, and Frederick Henry, Prince of Orange during the Siege of Groenlo (1627) and the Siege of 's-Hertogenbosch. As Stadholder of Groningen he founded the Nieuweschans fortress in 1628. Although he owned little in Friesland, he was popular there, and people granted his heir the right to rule after his death.

He was killed by a bullet at the Siege of Roermond while he was inspecting the trenches in June 1632. His son, Hendrik Casimir I succeeded him as count of Nassau-Dietz and as stadholder of Friesland, Groningen and Drenthe.

In 1607 Ernst Casimir married Sophia Hedwig of Brunswick-Lüneburg, daughter of Henry Julius, Duke of Brunswick-Lüneburg. From this marriage nine children were born:

• stillborn daughter (1608)
• stillborn son (1609)
• unnamed son (1610–1610)
• Hendrik Casimir I of Nassau-Dietz (Arnhem, 1612–1640)
• Willem Frederik of Nassau-Dietz (Arnhem, 1613–1664), married Albertine Agnes of Orange-Nassau.
• Elisabeth (Leeuwarden, 25 July 1614 – Leeuwarden, 18 September 1614)
• Johan Ernst (Arnhem, 29 March 1617 – May 1617)
• Maurice (Groningen, 21 February 1619 – Groningen, 18 September 1628)
• Elisabeth Friso (Leeuwarden, 25 November 1620 – Groningen, 20 September 1628)