Louis IX, Landgrave of Hesse-Darmstadt

Louis IX
Louis IX
Landgrave of Hesse-Darmstadt
Reign	12 October 1768 –; 6 April 1790
Predecessor	Louis VIII
Successor	Louis X
Born	15 December 1719; Darmstadt
Died	6 April 1790 (aged 70); Pirmasens
Spouses	; Countess Palatine Caroline of Zweibrücken (m. 1741; died 1774) ; Marie Adelaide of Cheirouze, Countess of Lemberg (m. 1775)
Issue	Caroline, Landgravine of Hesse-Homburg; Frederica Louisa, Queen of Prussia; Louis I, Grand Duke of Hesse; Amalie, Hereditary Princess of Baden; Wilhelmina Louisa, Tsarevna of Russia; Louisa Augusta, Grand Duchess of Saxe-Weimar-Eisenach; Prince Frederick; Prince Christian;
House	Hesse-Darmstadt
Father	Louis VIII, Landgrave of Hesse-Darmstadt
Mother	Countess Charlotte of Hanau-Lichtenberg

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 theosophy and alchemy. He corresponded with Emanuel Swedenborg in 1771, as Swedonborg hoped that Louis would take over 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.