Ephialtes of Trachis (, Ephialtēs; although Herodotus spelled it as , Epialtes) was the son of Eurydemus of Malis. He betrayed his homeland, in hope of receiving some kind of reward from the Persians, by showing the Persian forces a path around the allied Greek position at the pass of Thermopylae, which helped them win the Battle of Thermopylae in 480 BC.