An ephor was an official of ancient Sparta. There were five ephors elected annually, who swore each month to uphold the rule of the kings, while the kings swore to uphold the law.

Herodotus claimed that the institution was created by Lycurgus, but it may have arisen from the need for governors while the kings were leading armies in battle. The ephors were elected by the popular assembly, and all citizens were eligible for election. They provided a balance for the two kings, who rarely cooperated with each other. Plato called them tyrants who ran Sparta as despots, while the kings were little more than generals.

The ephors presided over meetings of the Gerousia, the oligarchic council of elders. They were in charge of civil trials, taxation, the calendar, foreign policy, and military training for young men. The year was named after one of them, like the eponymous archon of Athens. Two ephors accompanied the army in battle, and they could arrest and imprison the kings for misconduct during war. The ephors were also considered to be personally at war with the helots, so that they could imprison or execute any of them for any reason at any time without having to bring them to trial or violate religious rituals.

Cleomenes III abolished the ephors in 227 BC, but they were restored by the Macedonian king Antigonus III Doson after the Battle of Sellasia. The position existed into the 2nd century AD when it was probably abolished by the Roman emperor Hadrian.