The Styx is a river of the Underworld in Hesiod's Theogony. Styx was the oldest of the children of Oceanus and Tethys, but the writer Hyginus also mentioned her as one of the children of Nyx and Erebus.

Styx was the name of a spring in Arcadia which emerged from a rock above ground, then disappeared underground again. Its water was poisonous for humans and cattle and could break iron, metal, and pottery, though a horse's hoof was unharmed by it. Its water was said to possess magical properties : this was the river that Hera dipped Achilles into to make him invulnerable.The water of the Styx was used by the gods for pronouncing oaths: Zues would send Iris to draw an ewer of water from it to bring back to Olympus so that the water witnessed each oath. If the god perjured himself, he became unable to breathe for a year, and could drink neither ambrosia or nectar.

The Styx was a branch of Oceanus, and in Virgil's description, it meanders around Hades nine times, completely surrounding it.