Leonidas I (Doric Greek: Λεωνίδας, Leōnidas; Ionic Greek: Λεωνίδης, Leōnidēs; "son of the lion"; died 9 August 480 BCE)was a hero-king of Sparta, the 17th of the Agiad line, one of the sons of King Anaxandridas II of Sparta, who was believed in mythology to be a descendant of Heracles, possessing much of the latter's strength and bravery. Leonidas I is notable for his leadership at the Battle of Thermopylae.According to Herodotus, Leonidas' mother was his father's niece and had been barren for so long that the ephors, the five annually elected administrators of the Spartan constitution, tried to prevail upon King Anaxandridas to set aside his wife and take another. Anaxandridas refused, claiming his wife was blameless, whereupon the ephors agreed to allow him to take a second wife without setting aside his first. This second wife, a descendent of Chilon the Wise, promptly bore a son, Cleomenes. However, one year after Cleomenes' birth, Anaxandridas' first wife also gave birth to a son, Dorieus. Leonidas was the third son of Anaxandridas' first wife, and either the elder brother or twin of Cleombrotus.Because Leonidas was not heir to the throne, he was not exempt from attending the agoge, the public school that the sons of all Spartans had to complete in order to qualify for citizenship.Leonidas was thus one of the few Spartan kings to have ever undergone the notoriously harsh training of Spartan youth. Leonidas also went into the barracks to meet his future general Lysander. During this pre-teen time he was good friends with Lysander. Both of these boys grew up to be legends in the Spartan culture and history of Greece.

Cleomenes succeeded to his father's throne somewhere between 520 and 516 BCE.Dorieus was so outraged that the Spartans had preferred his half-brother over himself that he found it impossible to remain in Sparta. He made one unsuccessful attempt to set up a colony in Africa and, when this failed, sought his fortune in Sicily, where after initial successes he was killed.Leonidas' relationship with his bitterly antagonistic elder brothers is unknown, but he married Cleomenes' daughter, Gorgo sometime before coming to the throne in 490 BCE.Leonidas was clearly heir to the Agiad throne and a full citizen at the time of the Battle of Sepeia against Argos (c. 494 BC). Likewise, he was a full citizen when the Persians sought submission from Sparta and met with vehement rejection in or around 492/491 BCE. His elder brother the king had already been deposed on grounds of purported insanity, and had fled into exile when Athens sought assistance against the Persian invasion that ended at Marathon (490 BCE).

Plutarch has recorded the following: "When someone said to him: 'Except for being king you are not at all superior to us,' Leonidas son of Anaxandridas and brother of Cleomenes replied: 'But were I not better than you, I should not be king.'"As the product of the agoge, Leonidas is unlikely to have been referring to his royal blood alone but rather suggesting that he had, like his brother Dorieus, proven superior capability in the competitive environment of Spartan training and society, and that he believed this made him qualified to rule.

Leonidas was chosen to lead the combined Greek forces determined to resist the Persian invasion in 481 BCE. This was not simply a tribute to Sparta's military prowess: The probability that the coalition wanted Leonidas personally for his capability as a military leader is underlined by the fact that just two years after his death, the coalition preferred Athenian leadership to the leadership of either Leotychidas or Leonidas' successor (as regent for his still under-aged son) Pausanias. The rejection of Leotychidas and Pausanias was not a reflection on Spartan arms. Sparta's military reputation had never stood in higher regard. Nor was Sparta less powerful in 478 BCE than it had been in 481 BCE.

This election of Leonidas to lead the defense of Greece against Xerxes' invasion led to Leonidas' death in the Battle of Thermopylae in 480 BCE.