Robert was the oldest son of Guy of Dampierre from his first marriage with Matilda of Béthune. His father essentially transferred the reign of Flanders to him in November 1299, during his war with Philip IV of France. Robert of Béthune gained military fame in Italy, when he fought at the side of his father-in-law, Charles I of Sicily (1265–1268) against the last Hohenstaufens, Manfred and Conradin.
Together with his father he took part in 1270 in the Eighth Crusade, led by Saint Louis.
After his return from the Crusade he continued to be a loyal aid for his father, politically and militarily, in the fight against the attempts of the French King Philip IV the Fair to add Flanders to the French crown lands.