What was the name of the Spartan elite infantry unit that fought at Thermopylae?

The Spartan elite infantry unit at Thermopylae was the 300 Spartiates.

In 480 BCE, King Leonidas led 300 full Spartan citizens, known as Spartiates, at the pass of Thermopylae against the invading Persian army. They were accompanied by allied Greek troops, so the famous defense was not carried out by only 300 men. The number refers specifically to the selected Spartan contingent.

The 300 were chosen from the citizen body and were generally men with living sons, allowing their households to continue the citizen line if they died. They fought in the narrow pass with Greek hoplite equipment and formation tactics, taking advantage of the restricted terrain.

The unit is often confused with the Persian “Immortals,” an elite guard described by Herodotus. The Immortals belonged to the Persian army, whereas the 300 Spartiates were Spartan citizens. Thermopylae ended in Persian victory, but the defense became one of the best-known episodes of Greek resistance.