William I, Elector of Hesse

William I
Elector of Hesse
Painting of the Elector wearing the ensign of the Order of the Elephant
Born	3 June 1743
Died	27 February 1821 (age 77)
Spouse	Wilhelmina Caroline of Denmark and Norway
Issue	Marie Friederike, Duchess of Anhalt-Bernburg Karoline Amelie, Duchess of Saxe-Gotha-Altenburg Friedrich William II, Elector of Hesse
House	House of Hesse
Father	Frederick II, Landgrave of Hesse-Kassel
Mother	Princess Mary of Great Britain
Religion	Calvinism

William I, Elector of Hesse (German: Wilhelm I., Kurfürst von Hessen; 3 June 1743 – 27 February 1821) was the eldest surviving son of Frederick II, Landgrave of Hesse-Kassel (or Hesse-Cassel) and Princess Mary of Great Britain, the daughter of George II.

William was born in Kassel, Hesse in 1743. His father, landgrave Frederick II (who died in 1785), had in 1747 abandoned the family and reverted to Catholicism. In 1755 he formally annulled his marriage. William's grandfather, Landgrave William, granted the newly acquired principality of Hanau to his daughter-in-law and grandsons. Technically, young William became the reigning prince of Hanau, while under his mother's regency. The young prince William, together with his two younger brothers, lived with their mother, the landgravine Mary. From 1747 they were supported by Protestant relatives and moved to Denmark. There they lived with Mary's sister, Louise of Great Britain, and her family; Louise died in 1751.

On 1 September 1764, William married his first cousin, Wilhelmina Caroline of Denmark and Norway (1747–1820), who was the second surviving daughter of Frederick V of Denmark and Norway. They married at Christiansborg Palace and resided for two decades mostly in Denmark.

In 1785 they moved to Kassel when William succeeded to the landgraviate. During the lifetime of his father, William had already received the Principality of Hanau, south of the Hessian territories near Frankfurt, as successor of its newly extinct princes. The Hanau people did not want to have a Catholic ruler.

William's younger brother Charles in 1766 married another of their Danish first cousins, Princess Louise of Denmark.