| William VI | |
| Succession: | Landgrave of Hesse-Kassel |
| Reign: | 21 September 1637 – 16 July 1663 |
| Predecessor: | William V |
| Successor: | William VII |
| Issue: | Charlotte Amelie, Queen of Denmark William VII, Landgrave of Hesse-Kassel Luise Charles I, Landgrave of Hesse-Kassel Philip, Landgrave of Hesse-Philippsthal Georg Elisabeth Henriëtte, Electoral Princess of Brandenburg |
| House: | Hesse-Kassel |
| Father: | William V, Landgrave of Hesse-Kassel |
| Mother: | Amalie Elisabeth von Hanau-Münzenberg |
| Birth Date: | 23 May 1629 |
| Birth Place: | Kassel |
| Death Date: | 16 July 1663 (aged 34) |
| Death Place: | Haina |
Wilhelm VI, Landgrave of Hesse-Kassel (23 May 1629 – 16 July 1663), known as William the Just, was Landgrave of Hesse-Kassel from 1637 to 1663.
Born in Kassel, he was the son of William V (whom he succeeded) and his wife Amalie Elisabeth, Gräfin of Hanau-Münzenberg (daughter of Philip Louis II of Hanau-Münzenberg and his wife Countess Catharina Belgica of Nassau). His mother remained his guardian until he came of age. Despite Hesse-Kassel's defeat in the Thirty Years' War, William's mother did not wish to acknowledge the accord of 1627. This required that the unmarried Marburger heir and the Landgraves of Hessen-Darmstadt should fall, but Amalie Elisabeth had other ideas and led Hesse-Kessel in 1645 into the "Hessenkrieg", ruling as Landgräfin on her son's behalf. This war began when Hesse-Kassel's troops began to besiege the city of Marburg. Three years later, in 1648, the war ended with a victory for Kassel, although the citizens of Darmstadt also gained from it. Domination over the Marburger territories went over to the landgrave of Hesse-Kassel after the accord was dissolved and a new agreement was reached. William VI succeeded in what his ancestors had tried to do in vain since 1604, that is, to end the Hesse-Marburg landgraviate, and to annex the Marburger lands to Hesse-Kassel.
After these wars, William attended above all to the extension of the universities within his domains and the foundation of more new Lehranstalts. To finally resolve the quarrel with the landgraves of Hesse-Darmstadt, Wilhelm delivered to George II the territory around Gießen, along with Ämtern by Biedenkopf.
Shortly before his death, William joined the League of the Rhine on its foundation in 1658. He also sought to effect a union between his Lutheran and Reformed subjects, or at least to lessen their mutual hatred. In 1661 he had a colloquy held in Kassel between the Lutheran theologians of the University of Rinteln and the Reformed theologians of the University of Marburg.
William VI died at Haina in 1663. Control of his Landgraviate went to his eldest son William VII, though - not yet of age - he remained under the guardianship of his mother Hedwig Sophie of Brandenburg until his early death in 1670.
He married Margravine Hedwig Sophie of Brandenburg (1623–1683), daughter of daughter of George William, Elector of Brandenburg and Elizabeth Charlotte of the Palantine. Their children were:
|-