In Greek mythology, Helenus (; Ancient Greek: , Helenos, Latin: Helenus) was a son of King Priam and Queen Hecuba of Troy, and the twin brother of the prophetess Cassandra. He was also called Scamandrios. According to legend, Cassandra, having been given the power of prophecy by Apollo, taught it to her brother. Like Cassandra, he was always right, but unlike her, others believed him.
Helenus was part of the Trojan forces led by his brother Hector that beat the Greeks back from the plains west of Troy, and attacked their camp in the Iliad. When the Myrmidons led by Achilles turn the tide of battle and Hector is killed, foreshadowing Troy's imminent fall, Helenus - like most of the greatest heroes - survived the poem.
In the final year of the Trojan War, Helenus vied against his brother Deiphobus for the hand of Helen of Troy after the death of their brother Paris, but Helen was awarded to Deiphobus. Disgruntled over his loss, Helenus retreated to Mount Ida, where Odysseus later captured him. He told the Greek forces--probably out of his disgruntlement--under what circumstances they could take Troy. He said that they would win if they stole the Trojan Palladium, brought the bones of Pelops to Troy, and persuaded Neoptolemus (Achilles' son by the Scyrian princess Deidamia) and Philoctetes (who possessed Heracles' bow and arrows) to join the Greeks in the war. Neoptolemus was hiding from the war at Scyrus, but the Greeks retrieved him.
Neoptolemus had taken Andromache, Helenus's sister-in-law, and Hector's widow, as a slave and concubine after the fall of Troy, and fathered Molossus, Pielus and Pergamus with her. After the fall of Troy, Helenus went with Neoptolemus, according to Apollodorus' Epitome 6.13. He traveled with Neoptolemus, Andromache and their children to Epirus, where Neoptolemus permitted him to found the city of Buthrotum. After Neoptolemus left Epirus, he left Andromache and their sons in Helenus's care. Neoptolemus was killed by Orestes, Agamemmon's son, in dispute over Hermione, the daughter of Menelaus and Helen, whom Orestes had been promised as wife, but whom Neoptolemus had taken. As the kingdom of Neoptolemus was partitioned, this led to Helenus acquiring the rule of Buthrotum, as king. "Helenus, a son of Priam, was king over these Greek cities of Epirus, having succeeded to the throne and bed of Neoptolemus."  Andromache bore him a son, Cestrinus, who is identified with Genger or Zenter, a legendary Trojan king and father of Francus. Some mythographers alleged that Helenus married Neoptolemus's mother, Deidamia, as well as Andromache, in order to consolidate his claims on part of Neoptolemus' kingdom. Helenus prophesied Aeneas' founding of Rome when he and his followers stopped at Buthrotum, detailed by Virgil in Aeneid Book III.