Polydectes was the son of either Magnes and an unnamed Naiad, or of Peristhenes and Androthoe, or of Poseidon and Cerebia. His story is largely a part of the myth of Perseus, and runs as follows according to the Bibliotheca and John Tzetzes. He was the brother of the fisherman Dictys, who succeeded him on the throne.
Polydectes fell in love with Danaë when she and her son Perseus were saved by his brother Dictys (see: Acrisius). Perseus, old enough by the time, was very protective of his mother and would not allow Polydectes near her. Therefore, Polydectes hatched a plot to get him out of the way. Under the pretence that he was going to marry Hippodamia, he ordered every man in Seriphos to supply him with suitable gifts. His friends were to provide horses but Perseus failed to bring any, so Polydectes announced that he wanted nothing more than the head of the Gorgon Medusa, being that Perseus had previously said he was up to a task so harsh as fetching a Gorgon's head. Perseus agreed and Polydectes told him that he could not return to the island without it. Perseus slew Medusa, supposedly using his shield as a mirror to avoid looking at her.
When Perseus returned to Seriphos with the Gorgon's head, he found that, in his absence, his mother was threatened and abused (or even raped) by Polydectes, and had to seek refuge in a temple. Perseus was outraged and strode into the throne room where Polydectes and other nobles were convening. Polydectes was surprised that the hero was still alive and refused to believe Perseus had accomplished the deed he was sent out to do. Perseus professed that he did slay the Gorgon Medusa, and as proof, he revealed her severed head. When Polydectes and his nobles gazed upon the prize, they were then turned to solid stone. In a version recorded by Hyginus, Polydectes attempted to treacherously kill Perseus in fear of his courage, but Perseus was just in time to expose the Gorgon's head before him. Perseus then handed the kingdom of Seriphos over to Dictys.
In an alternate version followed by Hyginus, Polydectes married Danae as she was brought to him by Dictys, and had Perseus brought up in a temple of Athena. He did not abuse Perseus and Danae, but rather protected them from Acrisius as the latter discovered that they had survived and arrived at Seriphos to kill them. Perseus eventually swore to never kill his grandfather, but Polydectes soon died and at his funeral games Perseus accidentally hit Acrisius with a discus, which resulted in Acrisius' death.[a]