Syleus was a demigod and a son of Poseidon.
Syleus was the son of Poseidon, and was the brother of Dicaeus. He was a man of Aulis, Lydia, who was killed by Heracles for his nefarious deeds. In most versions Syleus owned a vineyard and forced all passers-by to dig it, which he might have attempted to do to Heracles as well. Heracles killed Syleus with the latter's own hoe and burned his vineyard down to the root. He also killed Syleus' daughter, Xenodoce.
Heracles gave hospitably to Dicaeus after the hero had done away with Syleus. Heracles fell in love with the daughter of Syleus who had been raised by her uncle, and married her. Soon, however, he left and his newlywed wife missed him so much that she died of grief. Upon return Heracles learned of her death and was about to throw himself onto her funeral pyre, but those present at the funeral ceremony managed to dissuade him from doing so. A temple of Heracles was erected next to her tomb.