When Neoptolemus I died in 360 BC, his brother Arymbas succeeded him on the Molossian throne. In 358 BC, Arymbas made a treaty with the new king of Macedonia, Philip II, and the Molossians became allies of the Macedonians. The alliance was cemented with a diplomatic marriage, when Arymbas' niece Olympias became Philip's wife in 357 BC and, consequently, queen consort of Macedonia. Philip had first fallen in love with Olympias when both were initiated into the mysteries of Cabeiri at the Sanctuary of the Great Gods, on the island of Samothrace.
One year later, in 356 BC, Philip's race horse won in the Olympic Games; for this victory his wife, who was known then as Myrtale, received the name Olympias. In the summer of the same year, Olympias gave birth to her first child Alexander. In ancient Greece people believed that the birth of a great man was accompanied by portents. As Plutarch describes, the night before the consummation of their marriage Olympias dreamed that a thunderbolt fell upon her womb and a great fire was kindled, its flames dispersed all about and then were extinguished. After the marriage Philip dreamed that he put a seal upon his wife's womb, the device of which was the figure of a lion. Aristander's interpretation was that Olympias was pregnant of a son whose nature would be bold and lion-like. Philip and Olympias also had a daughter, Cleopatra.
Their marriage was very stormy, Philip's volatility and Olympias' jealous temper had led to a growing estrangement. Things got even worse in 337 BC, when Philip married to a noble Macedonian woman, Cleopatra, who was niece of Attalus and after the marriage changed her name to Eurydice. This caused great tensions between Philip, Olympias and Alexander. Olympias went into voluntary exile in Epirus, staying at the Molossian court of her brother Alexander I who was the king at the time, along with her son Alexander who sided with her. In 336 BC, Philip cemented his ties to Alexander I of Epirus by offering him the hand of his and Olympias' daughter Cleopatra in marriage, a fact that led Olympias to further isolation as she could no longer count on her brother's support. However, Philip was murdered by his somatophylax Pausanias while attending the wedding and Olympias, who returned to Macedonia, was suspected of having countenanced his assassination.