The Roman Republic (Latin: Res publica Romana [ˈreːs ˈpuːblɪka roːˈmaːna]) was the era of classical Roman civilisation beginning with the overthrow of the Roman Kingdom (traditionally dated to 509 BC) and ending in 27 BC with the establishment of the Roman Empire following the War of Actium. During this period, Rome's control expanded from the city's immediate surroundings to hegemony over the entire Mediterranean world.

Roman society at the time was primarily a cultural mix of Latin and Etruscan societies, as well as of Sabine, Oscan, and Greek cultural elements, which is especially visible in the ancient Roman religion and its pantheon. Its political organisation developed at around the same time as direct democracy in ancient Greece, with collective and annual magistracies, overseen by a senate. There were annual elections, but the republican system was an elective oligarchy, not a democracy, with a number of powerful families largely monopolising the senior magistracies. Roman institutions underwent considerable changes throughout the Republic to adapt to difficulties such as the creation of promagistracies to rule its conquered provinces and, after the Social War during the late Republic, the inclusion of peninsular Italians as citizens and senators.

Unlike the Pax Romana of the Roman Empire, republican Rome was in a state of near-perpetual war. Its first enemies were its Latin and Etruscan neighbours, as well as the Gauls, who sacked Rome around 387 BC. After the Gallic sack, Rome conquered most of the Italian peninsula in a century and became one of the Mediterranean great powers. Its greatest strategic rival was Carthage, against which it waged three wars. Rome defeated Carthage at the Battle of Zama in 202 BC and in the following decades became the dominant power in the ancient Mediterranean world. It then embarked on a long series of difficult conquests, defeating Philip V and Perseus of Macedon, Antiochus III of the Seleucid Empire, the Lusitanian Viriathus, the Numidian Jugurtha, the Pontic king Mithridates VI, Vercingetorix of the Arverni tribe of Gaul, and the Egyptian queen Cleopatra.

At home, during the Conflict of the Orders, the patricians, the closed oligarchic elite, came into conflict with the more numerous plebs; this was resolved peacefully, with the plebs achieving political equality by the 4th century BC. The late Republic, from 133 BC onward, saw substantial domestic strife, often anachronistically seen as a conflict between optimates and populares, referring to conservative and reformist politicians, respectively. The Social War between Rome and its Italian allies over citizenship and Roman hegemony in Italy greatly expanded the scope of civil violence. Mass slavery also contributed to three Servile Wars. Political violence at home coupled with powerful semi-autonomous generals led to civil wars that broke the Republic. The first involved Marius and Sulla. After a generation, the Republic fell into civil war again in 49 BC between Julius Caesar and Pompey. Despite his victory and appointment as dictator for life, Caesar was assassinated in 44 BC. Caesar's heir Octavian and lieutenant Mark Antony defeated Caesar's assassins in 42 BC, but they split, eventually resulting in Antony's defeat alongside his ally and lover Cleopatra at the Battle of Actium in 31 BC. Octavian, the victor, then became the dominant force in Roman politics; Octavian's receipt of the title Augustus from the Senate in 27 BC is often seen to mark the end of the Republic and the beginning of the Roman Empire.

Rome had been ruled by monarchs since its foundation. These monarchs were elected, for life, by the men of the Roman Senate. The last Roman monarch was called Tarquin the Proud, who in traditional histories was expelled from Rome in 509 BC because his son, Sextus Tarquinius, raped a noblewoman, Lucretia. The tradition asserted that the monarchy was abolished in a revolution led by the semi-mythical Lucius Junius Brutus and the king's powers were then transferred to two separate consuls elected to office for a term of one year; each was capable of checking his colleague by veto. Most modern scholarship describes these accounts as the quasi-mythological detailing of an aristocratic coup within Tarquin's own family or a consequence of an Etruscan occupation of Rome rather than a popular revolution.

According to Rome's traditional histories, Tarquin made several attempts to retake the throne, including the Tarquinian conspiracy, which involved Brutus's own sons, the war with Veii and Tarquinii, and finally the war between Rome and Clusium. The attempts to restore the monarchy did not succeed.

The first Roman republican wars were wars of expansion. One by one, Rome defeated both the persistent Sabines and the local cities. Rome defeated its rival Latin cities in the Battle of Lake Regillus in 496 BC, the Battle of Ariccia in 495 BC, the Battle of Mount Algidus in 458 BC, and the Battle of Corbio in 446 BC. But it suffered a significant defeat at the Battle of the Cremera in 477 BC, wherein it fought against the most important Etruscan city, Veii; this defeat was later avenged at the Battle of Veii in 396 BC, wherein Rome destroyed the city. By the end of this period, Rome had effectively completed the conquest of its immediate Etruscan and Latin neighbours and secured its position against the immediate threat posed by the nearby Apennine hill tribes.

Beginning with their revolt against Tarquin, and continuing through the early years of the Republic, Rome's patrician aristocrats were the dominant force in politics and society. They initially formed a closed group of about 50 large families, called gentes, who monopolised Rome's magistracies, state priesthoods, and senior military posts. The most prominent of these families were the Cornelii, Aemilii, Claudii, Fabii, and Valerii. The leading families' power, privilege and influence derived from their wealth, in particular from their landholdings, their position as patrons, and their numerous clients.