Alexander I of Epirus (Ancient Greek: Ἀλέξανδρος Α' τῆς Ἠπείρου, 370 BC – 331 BC), also known as Alexander Molossus (Ἀλέξανδρος ὁ Μολοσσός), was a king of Epirus (350–331 BC) of the Aeacid dynasty.
As the son of Neoptolemus I and brother of Olympias, Alexander I was an uncle of Alexander the Great. He was also an uncle of Pyrrhus of Epirus. He was brought at an early age to the court of Philip II of Macedon, and after the Hellenic fashion became the object of his attachment. At the age of about 20, Philip 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.
Alexander, however, declined the contest, and formed a second alliance with Philip by taking to wife the daughter of Philip (Alexander'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 in the battle of Pandosia and was killed by a Lucanian. He left a son, Neoptolemus, and a daughter, Cadmea.
In a famous passage that is often considered the first specimen of alternative history, Livy speculates on what would have been the outcome of a military showdown between Alexander the Great and the Roman Republic. He reports there that as Alexander of Epirus lay mortally wounded on the battlefield at Pandosia he compared his fortunes to those of his famous nephew and said that the latter "waged war against women".