The Cantabri (Ancient Greek: Καντάβροι, Kantabroi) or Ancient Cantabrians were a pre-Roman people and large tribal federation that lived in the northern coastal region of ancient Iberia in the second half of the first millennium BC. These peoples and their territories were incorporated into the Roman Province of Hispania Tarraconensis in 19 BC, following the Cantabrian Wars.

Cantabri is a Latinized form of a local name, presumably meaning "Highlanders" and deriving from the reconstructed root *cant- ("mountain") in Ancient Ligurian. During the High and Late Middle Ages, as well as Modern Period, the name refers usually to the Basques.

Cantabria, the land of the Cantabri, originally comprised much of the highlands of the northern Spanish Atlantic coast, including the whole of modern Cantabria province, eastern Asturias, nearby mountainous regions of Castile and León, the northern of province of Palencia and province of Burgos and northeast of province of León. Following the Roman conquest, this area was, however, much reduced, making up only Cantabria and eastern Asturias.

The ancestors of the Cantabri were thought by the Romans to have migrated to the Iberian Peninsula around the 4th Century BC, and were said by them to be more mixed than most peninsular Celtic peoples. By the 1st century BC they comprised eleven or so tribes—Avarigines, Blendii [es], Camarici or Tamarici, Concani, Coniaci or Conisci, Morecani, Noegi, Orgenomesci, Plentuisii, Salaeni, Vadinienses, and Vellici or Velliques—gathered into a tribal confederacy with the town of Aracillum (Castro de Espina del Gallego, Sierra del Escudo – Cantabria), located at the strategic Besaya river valley, as their capital. Other important Cantabrian hillforts included Villeca/Vellica (Monte Cildá [es] – Palencia), Bergida (Castro de Monte Bernorio – Palencia) and Amaya/Amaia (Peña Amaya [es] – Burgos).

A detailed analysis of place-names in ancient Cantabria shows a strong Celtic element along with an almost equally strong "Para-Celtic" element (both Indo-European) and thus disproves the idea of a substantial pre-Indo-European or Basque presence in the region. This supports the earlier view that Untermann considered the most plausible, coinciding with archaeological evidence put forward by Ruiz-Gálvez in 1998, that the Celtic settlement of the Iberian Peninsula was made by people who arrived via the Atlantic Ocean in an area between Brittany and the mouth of the River Garonne, finally settling along the Galician and Cantabrian coast.

Regarded as savage and untamable mountaineers, the Cantabri long defied the Roman legions and made a name for themselves for their independent spirit and freedom. Indeed, Cantabri warriors were regarded as being tough and fierce fighters, suitable for mercenary employment, but prone to banditry.

The earliest references to them are found in the texts of ancient historians such as Livy and Polybius, who mention Cantabrian mercenaries in Carthaginian service in the late 3rd century BC. During the 2nd Punic War, a Cantabrian mercenary contingent is mentioned in Hannibal's army, whilst another Cantabri mercenary band led by a chieftain named Larus was recruited by Mago and fought in Celtiberia against the propraetor Marcus Junius Silanus in 207 BC. That same year, other Cantabrian mercenaries fought alongside the Astures' at the Battle of the Metaurus, and later Cantabrian war-bands fought for the Vaccaei and Celtiberians in the Celtiberian Wars of the 2nd century BC. Another author, Cornelius Nepos, claims that the Cantabrian tribes first submitted to Rome upon Cato the Elder's campaigns in Celtiberia in 195 BC. In any case, such was their reputation that when a battered Roman army under consul Gaius Hostilius Mancinus was besieging Numantia in 137 BC, the rumor of the approach of a large combined Cantabri-Vaccaei relief force was enough to cause the rout of 20,000 panic-stricken Roman legionaries, forcing Mancinus to surrender under humiliating peace terms.

In the early 1st century BC, the Cantabri began to play a double game by lending their services to individual Roman generals on occasion but, at same time, supported rebellions within Roman Spanish provinces and carried out raids in times of unrest. This opportunistic policy led the Cantabri to initially side with Quintus Sertorius during the Sertorian Wars, but at the final phase of the conflict they shifted their allegiance to Pompey, continuing to follow the Pompeian cause until the defeat of their generals' Lucius Afranius and Marcus Petreius at the battle of Ilerda (Lérida) in 49 BC. In between, the Cantabri had unsuccessfully intervened in the Gallic Wars by sending in 56 BC an allegedly 40,000-strong army to help the Aquitani tribes of south-western Gaul against the legate Publius Crassus, the son of Marcus Crassus serving under Julius Caesar, who succeeded in overpowering and destroying the combined Cantabri-Aquitani force of 50,000 men in their own camp and slaughtered 38,000 of them.