Pirithous was the king of the Lapiths in Thessaly and the husband of Hippodamia. His wedding began a battle between the Lapiths and the Centaurs.
Pirithous was a son of "heavenly" Dia and fathered by Zeus. His best friend was Theseus. Nestor numbers Pirithous and Theseus "of heroic fame" among an earlier generation of heroes of his youth, "the strongest men that Earth has bred, the strongest men against the strongest enemies, a savage mountain-dwelling tribe whom they utterly destroyed".
Pirithous had heard rumors about Theseus' courage and strength in battle but he wanted proof. He rustled Theseus' herd of cattle from Marathon, and Theseus set out to pursue him. Pirithous took up arms and the pair met, then became so impressed by each other they took an oath of friendship.
They were among the company of heroes that hunted the Calydonian Boar, another mythic theme that was already well-known to Homer's listeners. Later, Pirithous was set to marry Hippodamia, and had a son by her named Polypoetes. The centaurs were guests at the party, but they got drunk and tried to abduct the women, including Hippodamia. The Lapiths won the ensuing battle against the centaurs, the Centauromachy.
Theseus and Pirithous pledged to carry off daughters of Zeus. Theseus chose Helen of Sparta and together they kidnapped her when she was 13 years of age and decided to hold onto her until she was old enough to marry. Pirithous chose a more dangerous prize: Persephone herself. They left Helen with Theseus' mother, Aethra, and traveled to the underworld domain of Persephone and her husband Hades. When they stopped to rest, they found themselves unable to stand up from the rock as they saw the Furies appear before them.
Heracles freed Theseus from the stone, but the earth shook when he attempted to liberate Pirithous. He had committed too great a crime for wanting the wife of one of the great gods as his own bride. By the time Theseus returned to Athens, the Dioscuri (Helen's twin brothers Castor and Pollux) had taken Helen back to Sparta; they had taken captive Aethra and Physadeia, the sister of Pirithous, who became handmaidens of Helen and later followed her to Troy.