Thermopylae is a mountain pass in Greece. The name means roughly "hot gateway", named for several natural hot water springs there.

The pass runs from Locris into Thessaly between Mount Oeta and the sea (Maliac Gulf).

It is chiefly famous for the Battle of Thermopylae in 480 BC. Two other famous battles took place at the pass. In 279 BC Brennus and the Gauls were checked for several months by a Greek army under the Athenian Calippus, and in 191 BC Antiochus III the Great of Syria vainly attempted to hold the pass against the Romans under Manius Acilius Glabrio.

In the time of Leonidas in 480 BC the pass was a narrow track (probably about 14 yards wide) under the cliff. In modern times the deposits of the Spercheius have widened it to a breadth of 1 to 3 miles broad.

The hot springs from which the pass derived its name still exist close to the foot of the hill.