Alexander I of Epirus (362 BC ca.- 331 BC/330 BC), also known as Alexander Molossus, king of Epirus (343 BC/342 BC-331 BC/330 BC), of the Molossian dinasty.

He was son of king Neoptolemus, 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 BC) while Philip married his sister, Olympias.

In 343 BC he became king of Epirus.

In 333 BC, the same year in which Alexander the Great began his war against the Persian empire, Alexander Molossus was called by the Greek colony of Taras (Magna Graecia), to fight the Italic populations of Lucanians, Bruttii 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 BC (or earlier in 331 BC), he was defeated at Pandosia, near Cosenza, by the Lucanians and the Bruttii, and slain by a Lucanian emigrant.

See Justin viii. 6, ix. 6, xii. 2; Livy viii. 3, 17, 24; Aulus Gellius xvii. 21; and article Macedonian Empire.