Alcioneus (Greek Άλκυονεύς) in Greek mythology was one of the giants, son of Gaia (the Earth) and Uranus (the Sky). He was killed by Heracles when attacking his retinue after the tenth job - stealing Geryon's cattle.
Alcioneus had terrible strength and was one of the leaders of the Gigantomaquia. He had been wounded by an arrow from Heracles, but as soon as he fell to the ground he rose again, for he was immortal while remaining in his homeland, Palene (or, according to other versions, Phlegrae). Athena informed the hero of this, who dragged the giant out of his homeland, and so he died.
According to Robert Graves, the theft of Helios' cattle by Alcioneus is an older version of the myth of Heracles and the theft of Geryon's cattle.
It is further told that Alcioneus seized the isthmus of Corinth just as Heracles was fleeing with Geryon's cattle. He attacked the hero's entourage, hurling a huge boulder that smashed twelve carts and twenty-four of Heracles' men at once. The hero, with his club, struck back the rock with his club, thus killing the giant.
According to Pindar (Nemeas iv.44), the block with which the giant had attacked Heracles' life could still be seen on the isthmus until very recent times. In another passage (Isthmicas vi.45 y sig.), Pindar describes Alcioneus as a Thracian shepherd and places this fight on the plains of the Flégre Fields.
The twelve daughters of Alcioneus, the alcionidas, threw themselves into the sea in despair over the death of their father, and then became alciones.