Alexander I, was king of Epirus about 342 BC. He was the brother of Olympias, the mother of Alexander the Great, and both son-in-law and brother-in-law to Philip of Macedon, since he married Philip's daughter Cleopatra (336) while Philip married his sister, Olympias.
In 332 he crossed over to Italy to assist the Tarentines against the Lucanians, Bruttians and Samnites. He gained considerable successes and made an arrangement with the Romans for a joint attack upon the Samnites; but the Tarentines, suspecting him of the design of founding an independent kingdom, turned against him. Although the advantage at first rested with Alexander, he gradually lost it, and his supporters dwindled away. In 330 (or earlier) he was defeated at Pandosia and slain by a Lucanian emigrant.