Philip's first marriage was celebrated in Rome on February 12, 1301 to Isabella of Villehardouin, Princess of Achaea. By that marriage, he became Prince of Achaea, though he had already been lord of Piedmont by inheritance from his father in 1282. As prince, Philip ventured to reconquer all of Lacedaemonia from the Greeks. He was, however, an authoritative prince and this put him at odds with the baronage of his realm. He tried to placate the barons of Morea, but was forced to accept a parliament in 1304. The Greek peasantry, crushed by taxes, then revolted in turn. In 1307, King Charles II of Naples, the suzerain of Achaea, confiscated the principality and gave it to his son, Prince Philip I of Taranto.
In 1312, Philip married Catherine de la Tour du Pin (died 1337) and had issue: