Louis IX, Landgrave of Hesse-Darmstadt (b. 1719)
Louis IX of Hesse-Darmstadt (German: Ludwig) (15 December 1719 – 6 April 1790) was the Landgrave of Hesse-Darmstadt from 1768 - 1790. He was a son of Louis VIII, Landgrave of Hesse-Darmstadt, and Charlotte of Hanau-Lichtenberg and Müntzenberg.

He was born in Darmstadt on 15 December 1719. 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 (1759–1802)

Prince Christian of Hesse-Darmstadt (1763–1830)

A stillborn son on May 3, 1742In 1775, Louis married Marie Adelaide of Cheirouze, countess of Lemberg. On 6 April 1790 Louis died in Pirmasens.