Elis (/ˈiːlɪs/) or Eleia (/ɪˈlaɪ.ə/; Attic Greek: Ἦλις, romanized: Ēlis [ɛ̂ːlis]; Elean: Ϝᾶλις [wâːlis]; Greek: Ήλιδα, romanized: Ilida) was an ancient district in the northwestern Peloponnese in Greece, roughly corresponding to modern regional unit of Elis. It was bounded to the northeast by Achaea, east by Arcadia, south by Messenia, and west by the Ionian Sea. The region is a fertile lowland with extensive plains, watered by the Peneus, Alpheus and other rivers that flow down from the Arcadian highlands. The region's name is probably cognate with the English 'valley'.

The Eleans traced their roots back to the mythical Dorian invasion. They united into a single polis ("city-state") centred on the city of Elis in 471 BC. Over the course of the archaic and classical periods, the city gradually gained control of much of the region of Elis, most probably through unequal treaties with other cities; many inhabitants of Elis were Perioeci—autonomous free non-citizens. Elis' main claim to fame was its control over the sanctuary at Olympia and the Olympic games.

The region was probably not the site of a kingdom during the Mycenean period. According to myth, the original inhabitants of Elis were called Caucones and Paroreatae. The people of the region are mentioned by Homer for the first time in Greek history under the title of Epeians (Epeii), as setting out for the Trojan War, and they are described by him as living in a state of constant hostility with their neighbours the Pylians. In the mythical Dorian invasion of the Peloponnese, Elis was assigned to Oxylus and the Aetolians. The dialect spoken in Elis in historical times was Dorian, which might indicate that there had been an influx of people from northwestern Greece. These people, amalgamating with the Epeians, formed a powerful kingdom in the north of Elis. Three independent groups developed in the region: the Epeians, Minyae and Eleans.

Before the end of the 8th century BC, the Eleans had vanquished the Minyans and Epeians. Over the archaic period, they expanded their control of the region through conquest and treaties, reducing many of the surrounding communities to perioeci (non-citizen dependent communities). The Eleans enjoyed the support of Sparta in this process. The sanctuary of Olympia and the Olympic games, which were re-established in 776 BC, were initially controlled by the city of Pisa, but Elis contested this. They gained control of the sanctuary and games around 576 BC and destroyed the city of Pisa in 572 BC.

As an ally of Sparta, Elis joined the Peloponnesian League in the 6th century BC. Elis ostensibly supported the Greek side in the Persian Wars, but played no notable role in the conflict. In 472 BC, the literary sources report a revolution at Elis which established a democracy modelled on the Athenian one. This also entailed a synoecism, in which the various communities within the Elean orbit merged into a single political unit, with a capital city at Elis, located on the river Peneus, where it enters the lowland plain. Elis annexed most of Triphylia in the mid-fifth century BC, except for Lepreum in the south. Sometime afterwards, Lepreum was conquered as well.

When the Peloponesian War broke out between Athens and Sparta in 431 BC, Elis sided at first with Sparta. But the city state joined Argos and Athens in an alliance against Sparta in 421 BC due to Spartan support for the independence of Lepreum. The Eleans banned the Spartans from participating in the Olympic Games of 420 BC and defeated them in battle when they tried to participate anyway. The Eleans did not participate in the Battle of Mantinea in 418 BC, at which the Spartans definitively defeated the alliance. After the Peloponnesian War, Elis and Lepreum fought against Sparta in the Spartan-Elean War (402-400 BC), but they were defeated. As a result, in 399 BC, the Spartans forced the Eleans to rejoin the Peloponnesian League and made them give up Triphylia and the peroecic communities in Acroreia.

The Eleans attempted to re-establish their authority over these places after Thebes ended the Spartan hegemony at the Battle of Leuctra in 371 BC. They successfully retook the old peroecic communities in the north, but the newly established Arcadian League came to the assistance of the Triphylians in the south. After an oligarchic revolution in 365 BC, the Eleans launched a war with the Arcadians for the territory, which lasted until 363 BC. In the course of the war, the city of Elis was very nearly sacked by the Arcadians and the Eleans were forced to give up control of Olympia, but the Spartans came to their rescue and forced the Arcadians to withdraw. The territory of Triphylia was ceded to Arcadia.

Eric W. Robinson has argued that Elis was a democracy by around 500 BC, on the basis of early inscriptions which suggest that the people (the dāmos) could make and change laws. The literary sources refer to a democratic revolution in 472 BC. The literary sources imply that Elis continued to be democratic until 365, when an oligarchic faction took control. At some point in the mid-fourth century, democracy may have been restored; at least, we hear that a particularly narrow oligarchy was replaced by a new constitution designed by Phormio of Elis, a student of Plato. The classical democracy at Elis seems to have functioned mainly through a popular Assembly and a Council, the two main institutions of most poleis. The Council initially had 500 members, but grew to 600 members by the end of the fifth century. There was also a range of public officials such as the demiourgoi who regularly submitted to public audits.