Gaius Julius Caesar Augustus (23 September 63 BC – 19 August AD 14) was the first emperor of the Roman Empire, which he ruled alone from 31 BC until his death in AD 14. Born Gaius Octavius Thurinus, he was adopted posthumously by his great-uncle Gaius Julius Caesar in 44 BC, and between then and 31 BC was officially named Gaius Julius Caesar. In 27 BC the Senate awarded him the honorific Augustus ("the revered one"), and thus consequently he was Gaius Julius Caesar Augustus. Because of the various names he bore, it is common to call him Octavius when referring to events between 63 and 44 BC, Octavian (or Octavianus) when referring to events between 44 and 27 BC, and Augustus when referring to events after 27 BC. In Greek sources, Augustus is known as Ὀκτάβιος (Octavius), Καῖσαρ (Caesar), Αὔγουστος (Augustus), or Σεβαστός (Sebastos), depending on context.

The young Octavius came into his inheritance after Caesar's assassination in 44 BC. In 43 BC, Octavian joined forces with Mark Antony and Marcus Aemilius Lepidus in a military dictatorship known as the Second Triumvirate. As a triumvir, Octavian ruled Rome and many of its provinces as an autocrat, seizing consular power after the deaths of the consuls Hirtius and Pansa and having himself perpetually re-elected. The triumvirate was eventually torn apart under the competing ambitions of its rulers: Lepidus was driven into exile, and Antony committed suicide following his defeat at the Battle of Actium by the fleet of Octavian commanded by Agrippa in 31 BC.

After the demise of the Second Triumvirate, Octavian restored the outward facade of the Roman Republic, with governmental power vested in the Roman Senate, but in practice retained his autocratic power. It took several years to determine the exact framework by which a formally republican state could be led by a sole ruler; the result became known as the Roman Empire. The emperorship was never an office like the Roman dictatorship which Caesar and Sulla had held before him; indeed, he declined it when the Roman populace "entreated him to take on the dictatorship". By law, Augustus held a collection of powers granted to him for life by the Senate, including those of tribune of the plebs and censor. He was consul until 23 BC. His substantive power stemmed from financial success and resources gained in conquest, the building of patronage relationships throughout the Empire, the loyalty of many military soldiers and veterans, the authority of the many honors granted by the Senate, and the respect of the people. Augustus' control over the majority of Rome's legions established an armed threat that could be used against the Senate, allowing him to coerce the Senate's decisions. With his ability to eliminate senatorial opposition by means of arms, the Senate became docile towards his paramount position. His rule through patronage, military power, and accumulation of the offices of the defunct Republic became the model for all later imperial governments.

The rule of Augustus initiated an era of relative peace known as the Pax Romana, or Roman peace. Despite continuous frontier wars, and one year-long civil war over the imperial succession, the Mediterranean world remained at peace for more than two centuries. Augustus expanded the Roman Empire, secured its boundaries with client states, and made peace with Parthia through diplomacy. He reformed the Roman system of taxation, developed networks of roads with an official courier system, established a standing army (and a small navy), established the Praetorian Guard, and created official police and fire-fighting forces for Rome. Much of the city was rebuilt under Augustus; and he wrote a record of his own accomplishments, known as the Res Gestae Divi Augusti, which has survived. Upon his death in AD 14, Augustus was declared a god by the Senate, to be worshipped by the Romans. His names Augustus and Caesar were adopted by every subsequent emperor, and the month of Sextilis was officially renamed August in his honour. He was succeeded by his stepson and son-in-law, Tiberius.