Marie Louise of Hesse-Kassel was a Dutch regent, Princess of Orange by marriage to John William Friso, Prince of Orange, and regent of the Netherlands during the minority of her son and her grandson. She was a daughter of Charles I, Landgrave of Hesse-Kassel, and Maria Amalia of Courland. From 1939 to 1941 and again from 1943 to 2022, she and her husband were the most recent common ancestors of all currently reigning monarchs in Europe.

John William Friso became the (titular) Prince of Orange in 1702. He was the Stadtholder of Friesland and Groningen in the Dutch Republic until his death by accidental drowning in the Hollands Diep in 1711. From 1938 to 2022, Friso and his wife, Marie Louise, were the most recent common ancestors of all then-reigning European monarchs. As of 2023, the most recent common ancestors of all currently-reigning European monarchs are Louis IX, Landgrave of Hesse-Darmstadt and his wife Countess Palatine Caroline of Zweibrücken.