Landgrave William IX of Hesse-Kassel (1743-1821) was a German nobleman and a member of the House of Hesse. He ruled the Landgraviate of Hesse-Kassel from 1751 until his death, overseeing a period marked by economic development and military involvement in various European conflicts, including the Seven Years' War. William IX was also known for his patronage of the arts and sciences, contributing to the cultural landscape of his region. He was a significant figure in the German Enlightenment, promoting education and the establishment of institutions that fostered intellectual growth in Hesse-Kassel.