Alexander I of Epirus (Greek: Ἀλέξανδρος Α' της Ηπείρου, 370 - 331 BC), also known as Alexander Molossus (Greek: Ἀλέξανδρος ο Μολοσσός), was a king of Epirus (350 - 331 BC) of the Aeacid dynasty. As the son of Neoptolemus I and brother of Olympias, he was an uncle of Alexander the Great. He came at an early age to the court of Philip II of Macedon, and after the Grecian fashion became the object of his attachment. Philip in requital made him king of Epirus, after dethroning his uncle Arymbas. When Olympias was repudiated by her husband, 337 BC, she went to her brother, and endeavoured to induce him to make war on Philip.
Philip, however, declined the contest, and formed a second alliance with Alexander I by giving him his daughter (Alexander I's niece) Cleopatra in marriage (336 BC). At the wedding Philip was assassinated by Pausanias of Orestis. In 334 BC, Alexander I, at the request of the Greek colony of Taras (in Magna Graecia), crossed over into Italy, to aid them in battle against several Italic tribes, the Lucanians and Bruttii. After a victory over the Samnites and Lucanians near Paestum, 332 BC, he made a treaty with the Romans. Success still followed his arms. He took Heraclea from the Lucanians, and Terina and Sipontum from the Bruttii. Through the treachery of some Lucanian exiles, he was compelled to engage under unfavourable circumstances near Pandosia, on the banks of the Acheron, and was killed by the hand of one of the exiles, as he was crossing the river. He left a son, Neoptolemus, and a daughter, Cadmea.