Viriathus (also spelled Viriatus; known as Viriato in Portuguese and Spanish; died 139 BC) was the most important leader of the Lusitanian people that resisted Roman expansion into the regions of today known as Portugal, named Lusitania.

Viriathus developed alliances with other Celtic groups, even far away from his usual theatres of war, inducing them to rebel against Rome. He led his army, supported by most of the Lusitanian and Vetton tribes as well as by other Celtic and Iberian allies, to several victories over the Romans between 147 BC and 139 BC before being betrayed by them and murdered while sleeping. Theodor Mommsen said of him: "It seemed as if, in that thoroughly prosaic age, one of the Homeric heroes had reappeared."

There are several possible etymologies for the name Viriathus. The name can be composed of two elements: Viri and Athus. Viri may come from:

The Celtic elite used the title uiros ueramos, meaning the 'highest man' and the Latin equivalent would be summus vir.

According to the historian Schulten, Viriathus had a Celtic name.

For he was, as is agreed by all, valiant in dangers, prudent and careful in providing whatever was necessary, and that which was most considerable of all was, that whilst he commanded he was more beloved than ever any was before him.

— Diodorus Siculus

Little is known about Viriathus. The only reference to the location of his native tribe was made by the Greek historian Diodorus Siculus, who claims he was from the Lusitanian tribes of the ocean side.