The ephors were a board of five magistrates in ancient Sparta. They had an extensive range of judicial, religious, legislative, and military powers, and could shape Sparta's home and foreign affairs.

The word "ephors" (Ancient Greek ἔφοροι éphoroi, plural form of ἔφορος éphoros) comes from the Ancient Greek ἐπί epi, "on" or "over", and ὁράω horaō, "to see", i.e., "one who oversees" or "overseer". The ephors were a council of five Spartan men elected annually who swore an oath monthly on the behalf of the state. The Spartan kings, however, would swear on behalf of themselves.

The ephors did not have to kneel before the Kings of Sparta, and were held in high esteem by the citizens because of the importance of their powers and because of the holy role that they earned throughout their functions.

Several other Greek city-states with a Spartan ancestry also had ephors, such as Taras or Cyrene.

Two different accounts of the origins of the ephorate exist in ancient sources. The earliest account is found in the Histories of Herodotus, who traces its origins to the mythical Spartan lawgiver Lycurgus—a version followed by Xenophon, Plato, or Isocrates. A diverging version first appears in the Politics, written in the middle of the 4th century BC by Aristotle, who tells that the ephorate was created by the Spartan king Theopompos. This version is then more prevalent in subsequent authors, such as Cicero, and especially Plutarch. Modern scholars have identified the source of the second version in a lost work written by the Agiad king Pausanias after he had been forced to abdicate and go into exile in 394 BC. In this logos, Pausanias likely published Lycurgus' laws, including the Rhetra, which details the different element of the Spartan constitution (kings, gerousia, ekklesia), but does not mention the ephors. It has therefore been suggested that Pausanias was hostile to the ephors, to whom he possibly attributed his banishment, and published the Rhetra to discredit their office. Although the contents of this logos and Pausanias' motivations remain disputed, most modern scholars think the ephors were created at the time of Theopompos, during the Messenian Wars.

According to Plutarch, the ephorate was born out of the necessity for leaders while the kings of Sparta were absent for long periods during the Messenian Wars. The ephors were elected by the popular assembly, and all citizens were eligible. The position of ephor was the only political office open to the whole damos (populace) of men between the ages of 30–60, so eligible Spartans highly sought after the position. They were forbidden to be re-elected and provided a balance for the two kings, who rarely co-operated. Plato called the ephors tyrants, who ran Sparta as despots while the kings were little more than generals. Up to two ephors would accompany a king on extended military campaigns as a sign of control, and they held the authority to declare war during some periods in Spartan history.

Since political and economic decisions were made by majority vote, Sparta's policy could change quickly, when the vote of one ephor changed. For example, in 403 BC, Pausanias convinced three of the ephors to send an army to Attica, a complete reversal of the policy of Lysander. According to Aristotle, the ephors frequently came from poverty because any Spartan citizen could hold the position, and it was not exclusive to the upper-class. Aristotle stated that because of this they were often liable to corruption. There were times when the legal power of an ephor was taken advantage of, such as with Alcibiades's use of Endius, who persuaded the Spartans to allow Alcibiades to take control of Sparta's peace mission to Athens in 420 BC.

Cleomenes III abolished the position of ephor in 227 BC, and replaced them with a position called the patronomos. Cleomenes's coup resulted in the death of four of the five ephors, along with ten other citizens. His abolition of the ephorship allowed him to cement his role as king and prevent anyone from stopping his political reforms. However, the ephorate was restored by the Macedonian King Antigonus III Doson after the Battle of Sellasia in 222 BC. Although Sparta fell under Roman rule in 146 BC, the position existed into the 2nd century AD, when it was likely abolished by Roman Emperor Hadrian and superseded by imperial governance as part of the province of Achaea.