Quintus Sertorius (c. 126 BC – 73 or 72 BC) was a Roman general and statesman who led a large-scale rebellion against the Roman Senate on the Iberian Peninsula. Defying the regime of Sulla, Sertorius became the independent ruler of Hispania for most of a decade until his assassination.

Sertorius first became prominent during the Cimbrian War fighting under Gaius Marius, and then served Rome in the Social War. After Lucius Cornelius Sulla blocked Sertorius' attempt at the plebeian tribunate c. 88 BC, following Sulla's consulship, Sertorius joined with Cinna and Marius in the civil war of 87 BC. He led in the assault on Rome and restrained the reprisals that followed. During Cinna's repeated consulships he was elected praetor, likely in 85 or 84 BC. He criticised Gnaeus Papirius Carbo and other Marians' leadership of the anti-Sullan forces during the civil war with Sulla and was, late in the war, given command of Hispania.

In late 82 BC Sertorius was proscribed by Sulla and forced from his province. However, he soon returned in early 80 BC, taking in and leading many Marian and Cinnan exiles in a prolonged war, representing himself as a Roman proconsul resisting the Sullan regime at Rome. He gathered support from other Roman exiles and the native Iberian tribes – in part by using his tamed white fawn to claim he had divine favour – and employed irregular and guerrilla warfare to repeatedly defeat commanders sent from Rome to subdue him. He allied with Mithridates VI of Pontus and Cilician pirates in his struggle against the Roman government.

At the height of his power, Sertorius controlled nearly all of Hispania. He sustained his anti-Sullan resistance for many years, despite substantial efforts to subdue him by the Sullan regime and its generals Quintus Caecilius Metellus Pius and Pompey. After defeating Pompey in 76 BC at the Battle of Lauron however, he suffered repeated setbacks in later years. By 73 BC his allies had lost confidence in his leadership; his lieutenant Marcus Perperna Veiento assassinated him in 73 or 72 BC. His cause fell in defeat to Pompey shortly thereafter. The Greek biographer and essayist Plutarch chose Sertorius as the focus of one of his biographies in Parallel Lives, where he was paired with Eumenes of Cardia, one of the post-Alexandrine Diadochi.

Sertorius was born in Nursia in Sabine territory around 126 BC. The Sertorius family were of equestrian status. It appears that he did not have any noteworthy ancestors and was thus a novus homo (a "new man"), ie the first of his family to join the Senate. Sertorius' father died before he came of age and his mother, Rhea, focused all her energies on raising her only son. She made sure he received the best education possible for a young man of his status. In return, according to Plutarch, he became excessively fond of his mother. Having inherited his father's clients, like many other young rural aristocrats (domi nobiles), Sertorius sought to begin a political career and thus moved to Rome in his mid-to-late teens trying to make it big as an orator and jurist.

Sertorius' style of rhetoric was "blunt" but effective and bold. He made a sufficient impression on the young Cicero to merit a special mention in a later treatise on oratory, in which Cicero describes Sertorius' speaking style as talented but unpolished:

But of all that class of orators, or rather ranters, who were quite without training, without manners, or flatly uncouth, I hold Quintus Sertorius of our order, and Gaius Gargonius of the equestrian, to have been the readiest and shrewdest I have ever known.

After his undistinguished career in Rome as a jurist and an orator, he entered the military. Sertorius' first recorded campaign was under Quintus Servilius Caepio as a staff officer and ended at the Battle of Arausio in 105 BC, where he showed unusual courage. When the battle was lost, Sertorius escaped while wounded by swimming across the Rhone, apparently still with his weapons and armour. This became a minor legend in antiquity, still remembered in the time of Ammian.