| Prince William | |
| Issue: | Princess Karoline Marie Luise, Princess Frederick Augustus of Anhalt-Dessau Louise, Queen of Denmark Friedrich Wilhelm, Landgrave of Hesse Auguste, Baroness Carl Frederik of Blixen-Finecke Princess Sophie Wilhelmine |
| House: | Hesse-Kassel |
| Father: | Prince Frederick of Hesse-Kassel |
| Mother: | Princess Caroline of Nassau-Usingen |
| Birth Date: | 24 December 1787 |
| Birth Place: | Biebrich, Landgraviate of Hesse-Kassel |
| Death Place: | Copenhagen, Kingdom of Denmark |
Prince William of Hesse-Kassel (24 December 1787 – 5 September 1867) was the first son of Prince Frederick of Hesse-Kassel and Princess Caroline of Nassau-Usingen. He was titular Landgrave of Hesse-Kassel-(Rumpenheim)and for many years heir presumptive to the throne of Hesse-Kassel.
Prince William was born in Biebrich on Christmas Eve in 1787, the son of Prince Frederick of Hesse-Kassel and Princess Caroline of Nassau-Usingen. His father was himself a younger son of Frederick II, Landgrave of Hesse-Kassel and Princess Mary of Great Britain.
Prince William lived most of his life in Denmark where he had several posts in the Danish military. He was military governor of Copenhagen from 1834 until 1848. He lived for many years in Prince William Mansion, Copenhagen and afterwards in Brockdorff's Palace part of Amalienborg Palace, the official residence for the Danish royal family. As a summer residence the family used Charlottenlund Palace outside of Copenhagen.
Besides his Danish residences, he also had part ownership of the family castle, Rumpenheim Castle. The castle served as a meeting place for the Hesse-Kassel-Rumpenheim branch for many years and gathered Danish, Hesse, British and other royal families.
Landgrave William took a keen interest in Danish affairs of state and in the succession issues in the 1840-ties. He and his wife Charlotte were among the leading figures in Denmark, especially after his brother-in-law, Christian VIII of Denmark became king in 1839.
Landgrave William was known for his ultra-conservatism, but also for his hospitality, honesty, loyalty, and generosity.
He died in 1867 in Copenhagen, the year after Hesse-Kassel was annexed to Prussia. He was buried at Rumpenheim Castle.
On 10 November 1810, William was married in Amalienborg Palace to Princess Louise Charlotte of Denmark (1789–1864) daughter of Hereditary Prince Frederick of Denmark and Norway (1753–1805) and Sophia Frederica of Mecklenburg-Schwerin (1758–1794).
Children of the marriage:
Landgrave Wilhelm is the maternal grandfather of Queen Alexandra of the United Kingdom, King Frederick VIII of Denmark, King George I of Greece, Empress Maria Feodorovna of Russia, and Grand Duchess Adelheid of Luxembourg and paternal grandfather of Prince Frederick Charles of Hesse (elected King of Finland on 9 October 1918, but renounced the throne on 14 December 1918).