According to Herodotus, 400 Corinthian hoplites fought at Thermopylae.
The Corinthians were one of several Peloponnesian contingents that joined Leonidas’s force at the pass in 480 BC. Herodotus’s listing of the Greek troops includes 400 Corinthians, alongside 500 men from Tegea, 500 from Mantinea, 200 from Phlius, and 80 from Mycenae. These figures describe hoplite contingents, the heavily armed infantry that formed the core of Greek battlefield tactics.
The Corinthian unit was part of the larger Greek defense rather than the small Spartan force highlighted by later popular retellings. Leonidas initially commanded more than 7,000 Greek troops, though most were dismissed after the Persians discovered a route around the pass. The final stand included Spartans, Thespians, and Thebans; the Corinthian contingent was not among the groups that remained for that last phase.
Numbers can vary in summaries because ancient authors give different totals and because some accounts count helots or other supporting personnel separately. The answer remains 400 for Herodotus’s specific troop list.