Pirithous was a son of "heavenly" Dia, fathered either by Ixion or by Zeus. He married Hippodamia, daughter of Atrax or Butes, at whose wedding the famous Battle of Lapiths and Centaurs occurred. By his wife, he became the father of Polypoetes, one of Greek leaders during the Trojan War. Peirithous was also the close friend of the hero Theseus.
According to Homer, Dia had sex with Zeus, who was disguised as a stallion, and gave birth to Pirithous; a folk etymology derived Pirithous' name from peritheein (περιθεῖν "to run around"), because that was what Zeus did to seduce Dia.
His best friend was Theseus. In Iliad I, 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". No trace of such an oral tradition, which Homer's listeners would have recognized in Nestor's allusion, survived in literary epic.
In disjointed episodes that have survived, 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 (offspring: Polypoetes). The centaurs were guests at the party, but they got drunk and tried to abduct the women, including Hippodamia who was carried off by the intoxicated centaur Eurytion or Eurytus. The Lapiths won the ensuing battle, the Centauromachy, a favorite motif of Greek art.