Louis IX, Landgrave of Hesse-Darmstadt

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.

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