What naval battle was fought simultaneously with the Battle of Thermopylae?

The naval battle fought simultaneously with the Battle of Thermopylae was the Battle of Artemisium. In 480 BC, the Greek alliance tried to stop Xerxes I's invasion through a coordinated land-and-sea strategy: hoplites held the narrow pass at Thermopylae while the allied fleet confronted the Persian navy at Artemisium, off northern Euboea.

The naval engagement lasted three days and consisted of several clashes rather than one uninterrupted battle. The Greek fleet, commanded overall by the Spartan Eurybiades with the Athenian Themistocles playing a major strategic role, faced a much larger Persian force. Storms had already damaged many Persian ships before the main fighting began.

The two battles were closely connected. When the Greeks learned that Leonidas and the defenders at Thermopylae had been defeated, the fleet withdrew from Artemisium to avoid being trapped. The Persian victory there was therefore incomplete: the Greek navy survived to fight at Salamis, where it achieved a decisive victory later in the same year. Artemisium is sometimes confused with Salamis, but Salamis occurred afterward, not simultaneously.