Louis IX of Hesse-Darmstadt (German: Ludwig; 15 December 1719 – 6 April 1790) was Landgrave of Hesse-Darmstadt from 1768 to 1790.
After his military service in the army of Frederick the Great, Louis was involved in freemasonry and interested in theosophy and alchemy. He corresponded with Emanuel Swedenborg in 1771, as Swedonborg hoped that Louis would take over the leadership of Swedish Rite freemasonry in Germany.
As of 2026, Louis, along with his wife Caroline, are the most recent common ancestors of all currently reigning hereditary monarchs in Europe. The previous holder of this title was John William Friso, Prince of Orange.
Louis IX was born in Darmstadt on 15 December 1719 to Louis VIII, Landgrave of Hesse-Darmstadt, and Charlotte of Hanau-Lichtenberg and Müntzenberg.
His main residence was his palace in Pirmasens, Schloss Pirmasens.
On 12 August 1741, Louis married Caroline, daughter of Christian III, Duke of Zweibrücken. They had three sons and five daughters, including:
Princess Caroline of Hesse-Darmstadt (1746–1821), married Frederick V, Landgrave of Hesse-Homburg
Princess Frederica Louisa of Hesse-Darmstadt (1751–1805), married King Friedrich Wilhelm II of Prussia and became Queen of Prussia
Prince Louis X (1753–1830), later Grand Duke Louis I, married his first cousin Princess Louise of Hesse-Darmstadt
Princess Amalie of Hesse-Darmstadt (1754–1832), married her first cousin Karl Ludwig, Hereditary Prince of Baden
Princess Wilhelmina Louisa of Hessen-Darmstadt (1755–1776), married Grand Duke Pavel Petrovich of Russia, later emperor
Princess Luise Auguste of Hesse-Darmstadt (1757–1830), married Karl August, Grand Duke of Saxe-Weimar-Eisenach
Prince Frederick (10 June 1759 – 11 March 1802) Never married and had no issue.
Prince Christian of Hesse-Darmstadt (1763–1830)
In 1775, Louis IX married Marie Adelaide of Cheirouze, countess of Lemberg. On 6 April 1790, Louis died in Pirmasens.
Additionally Louis had an extramarital son with Ernestine Rosine Doll:
Ernest Louis von Hessenzeig (10 November 1761 - 22 December 1774) Died young.
John William Friso and Marie Louise of Hesse-Kassel – Most recent common ancestors of all reigning hereditary European monarchs from the end of World War II until 2022