The Lusitanians were an Indo-European-speaking people living in the far west of the Iberian Peninsula, in present-day central Portugal and the regions of Extremadura and Castilla y León of Spain. It is uncertain whether the Lusitanians were Celts or Celticized Iberians, related to the Lusones. After its conquest by the Romans, the land was subsequently incorporated as a Roman province named after them (Lusitania).

Frontinus mentions Lusitanian leader Viriathus as the leader of the Celtiberians, in their war against the Romans. The Lusitanians were also called Belitanians, according to the diviner Artemidorus. Strabo differentiated the Lusitanians from the Iberian tribes and thought of them as being Celtiberians who had been known as Oestriminis in ancient times. However, based on archeological findings, Lusitanians and Vettones seem to have been largely pre-Celtic Indo-European populations that adopted Celtic cultural elements by proximity. On the other hand, Pliny the Elder and Pomponius Mela distinguished the Lusitanians from neighboring Celtic tribes in their geographical writings.

The original Roman province of Lusitania briefly included the territories of the Astures and Gallaeci in the north, but these were soon ceded to the jurisdiction of the Provincia Tarraconensis, while the south remained the Provincia Lusitania et Vettones. Later, Gallaecia would become its own province. After this, Lusitania's northern border was along the Douro River, while its eastern border passed through Salmantica and Caesarobriga to the Anas (Guadiana) river.

Lusitanian mercenaries fought for the Carthaginian Empire between the years 218 and 201 BCE, during the Second Punic War against the Roman Republic that took place in the Western Mediterranean. Roman senator and orator Silius Italicus describes them in his 17-volumes epic poem Punica as forming a combined force with the Gallaeci and both being led by a commander named Viriathus (not to be confused with the similarly named chieftain). According to Roman historian Titus Livius, Lusitanian and Celtiberian cavalry performed raids in northern Italy whenever the terrain was too rough for the Carthaginian general Hannibal's famed Numidian cavalry.

Starting in 193 BCE, the Lusitanians fought the Romans in Hispania. In 150 BCE, they were defeated by the Roman praetor Servius Galba: springing a treacherous trap, he killed 9,000 Lusitanians and later sold 20,000 more as slaves in Gaul (modern-day France). This massacre would not be forgotten by Viriathus, who three years later (147 BCE) would become the leader of the Lusitanians, and severely damaged the Roman rule in Lusitania and beyond. In 139 BCE, Viriathus was betrayed and killed in his sleep by three of his companions (who had been sent as emissaries to the Romans), Audax, Ditalcus and Minurus, bribed by Marcus Popillius Laenas (although they were warrior companions of Viriathus, they were not Lusitanians themselves; they seem to have been Turdetanians, or from another people who were not Lusitanian). However, when the three returned to receive their reward from the Romans, the consul Quintus Servilius Caepio ordered their execution, declaring: "Rome does not pay traitors".

After the death of Viriathus, the Lusitanians kept fighting under the leadership of Tautalus, but gradually acquired Roman culture and language; the Romanized Lusitanian cities, in a manner similar to those of the rest of the Iberian Peninsula, eventually gained the status of "Citizens of Rome".

Categorising Lusitanian culture generally, including the language, is proving difficult and contentious. Some believe it was essentially a pre-Celtic Iberian culture with substantial Celtic influences, while others argue that it was an essentially Celtic culture with strong indigenous pre-Celtic influences associated with the Bell Beaker culture.

The Lusitanians worshiped various gods in a very diverse polytheism, using animal sacrifice. They represented their gods and warriors in rudimentary sculpture.