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 of Zweibrücken, 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
• Princess Amalie of Hesse-Darmstadt (1754–1832), married 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)

In 1775, Louis married Marie Adelaide of Cheirouze, countess of Lemberg. On 6 April 1790 Louis died in Pirmasens.