Henry Casimir I of Nassau-Dietz (21 January 1612 – 13 July 1640) was count of Nassau-Dietz and Stadtholder of Friesland, Groningen and Drenthe.

He was born in Arnhem, the eldest son of Ernst Casimir of Nassau-Dietz and Sophia Hedwig of Brunswick-Lüneburg, and, like his father, died in battle.

Henry Casimir was christened at Arnhem. The English representative, Sophia Hedwig being the niece of the queen Anne of Denmark, was Sir Edward Cecil. He brought gifts of a cupboard of gilt plate, a diamond necklace with a locket, horses, and an embroidered petticoat for Sophia Hedwig.^[1]

He became count of Nassau-Dietz and stadtholder of Friesland, Groningen and Drenthe upon the death of his father, Count Ernst Casimir of Nassau-Dietz, at the Siege of Roermond in June 1632. A week later, he was involved in the Capture of Maastricht, along with his cousin, Frederick Henry, Prince of Orange.

Henry Casimir was wounded in Sint Jansteen at the battle of Hulst on 12 July 1640. He died the next day and was buried in Leeuwarden. He was succeeded by William Frederick, Prince of Nassau-Dietz. His death in at the Battle of Hulst, aged 28, prompted the creation of several memorials to him and the battle. The Rijksmuseum collection contains a blood-stained shirt he is purported to have been wearing when he was wounded. The same collection also contains his father's hat, which has a bullet hole in it.^[2]