Henry Casimir I of Nassau-Dietz (Dutch: Hendrik Casimir I van Nassau-Dietz; Arnhem, 21 January 1612 – Hulst, 13 July 1640) was count of Nassau-Dietz and Stadtholder of Friesland, Groningen and Drenthe. He was the eldest son of Ernst Casimir of Nassau-Dietz and Sophia Hedwig of Brunswick-Lüneburg, and like his father, died in battle. On 12 July 1640, he was wounded in Sint Jansteen at the battle of Hulst. He died the next day. Hendrik Casimir is buried in Leeuwarden, and was succeeded in his titles by William Frederick, Prince of Nassau-Dietz. His death at age 28 caused a series of memorials to his name and the battle in which he died.

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