Leonidas I (died 19 September 480 BC) was a king of the Greek city-state of Sparta, and the 17th of the Agiad line, a dynasty which claimed descent from the mythical demigod Heracles.
Leonidas I was a son of King Anaxandridas II. He succeeded his half-brother King Cleomenes I to the throne in c. 489 BC. His co-ruler was King Leotychidas. He was succeeded by his son, King 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 in the battle. Leonidas entered myth as a hero and the leader of the 300 Spartans. While the Greeks lost this battle, they were able to expel the Persian invaders in the following year.