| Leonidas I | |
|---|---|
 Monument of Leonidas | |
| King of Sparta | |
| Reign | 489–480 BC |
| Predecessor | Cleomenes I |
| Successor | Pleistarchus |
| Born | c. 540 BC Sparta, Greece |
| Died | 11 August 480 BC (59–60) Thermopylae, Greece |
| Consort | Gorgo |
| Issue | Pleistarchus |
| Greek | Λεωνίδᾱς |
| House | Agiad |
| Father | Anaxandridas II |
| Religion | Greek polytheism |
Leonidas I (/liəˈnaɪdəs, -dæs/; ‹See Tfd›Greek: Λεωνίδας, Leōnídas; born c. 540 BC; died 11 August 480 BC) was king of the Ancient Greek city-state of Sparta. He was the son of king Anaxandridas II and the 17th king of the Agiad dynasty, a Spartan royal house which claimed descent from the mythical demigod Heracles. Leonidas I ascended to the throne in c. 489 BC, succeeding his half-brother king Cleomenes I. He ruled jointly along with king Leotychidas until his death in 480 BC, when he was succeeded by his son, Pleistarchus.
At the Second Greco-Persian War, Leonidas led the allied Greek forces in a last stand at the Battle of Thermopylae (480 BC), attempting to defend the pass from the invading Persian army, and was killed early during the third and last day of the battle. Leonidas entered myth as a hero and the leader of the 300 Spartans who died in battle at Thermopylae. While the Greeks lost this battle, they were able to expel the Persian invaders in the following year.