Ariarathes III (reigned 262 BCE or 255 BCE – 220 BCE), son of Ariamnes, ruler of Cappadocia, and grandson of Ariarathes II, marries Stratonice, a daughter of Antiochus II, king of Syria and wife Laodice I, and obtains a share in the government during the lifetime of his father.
About 250 BCE, he is the first ruler of Cappadocia to proclaim himself king (basileus).
It is known that he sided with Antiochus Hierax in his war against Seleucus II Callinicus.
Ariarathes is also said to have expanded his kingdom adding Cataonia to his dominions.
By his marriage, he is the father of Ariarathes IV, who succeeds him.