Philip III became king of France in August 1270 when Louis IX died during the Eighth Crusade at Tunis. He inherited an army weakened by disease and returned to France after a negotiated settlement. Royal policy was shaped by advisers and household figures, notably the chamberlain Pierre de la Broce. Philip's second marriage to Marie of Brabant in 1274 altered the balance at court; La Broce fell from favor and was executed in 1278.

The death of Philip's uncle Alphonse of Poitiers in 1271 brought major southern territories, including Toulouse, into the royal domain. Philip continued his father's administrative work and negotiated adjustments with Edward I over lands in southwestern France. The marriage of his son, the future Philip IV, to Joan of Navarre in 1284 connected the dynasty with Navarre and Champagne, extending its influence through inheritance and marriage as well as direct territorial acquisition.

Philip supported his uncle Charles of Anjou after the Sicilian Vespers disrupted Angevin rule in Sicily. With papal backing he undertook the Aragonese Crusade in 1285, seeking to enforce the dynasty's claims against Peter III of Aragon. The invading army achieved initial gains but suffered from disease, supply difficulties and naval defeat. Its retreat ended in failure, and Philip died at Perpignan in October. His son succeeded to a larger royal inheritance but also to the financial and political burdens of the unsuccessful campaign.