Robert I, known as Robert the Frisian, was count of Flanders from 1071 to his death in 1093. He was the son of Baldwin V, Count of Flanders and the younger brother of Baldwin VI, Count of Flanders. He usurped the countship after defeating his nephew Arnulf III and his allies, which included King Philip I of France, count Eustace of Boulogne and the counts of Saint-Pol and Ardres at the battle of Cassel. He subsequently made peace with Philip, who became his stepson-in-law, but remained hostile to his sister Matilda and brother-in-law, King William I of England.