Philippa was born in Valenciennes in the County of Hainaut in the Low Countries, a daughter of William I, Count of Hainaut, and Joan of Valois, Countess of Hainaut, granddaughter of Philip III of France. She was one of eight children and the second of five daughters. Her eldest sister Margaret married the German king Louis IV in 1324; and in 1345, she succeeded their brother William II, Count of Hainaut, upon his death in battle. William's counties of Zealand and Holland as well as of the seigniory of Frieze were devolved to Margaret after agreement between Philippa and her sister. Edward III of England, however, in 1364–65, in the name of his wife Philippa, demanded the return of Hainaut and other inheritances which had been given over to the Dukes of Bavaria–Straubing. He was not successful, as it was the custom in these regions to favour male heirs.