Ernst Casimir I of Nassau-Dietz was count of Nassau-Dietz and Stadtholder of Friesland, Groningen and Drenthe. He was the eleventh 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-Diez and Vianden were divided among his five living sons. Ernst Casimir followed him as Count of Nassau-Dietz in Diez, Germany. In 1631 he also inherited the small county of Spiegelberg (near Lauenstein).
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.