Augustus (Imperator Caesar Divi F. Augustus, 23 September 63 BC – 19 August 14 AD) was the founder of the Roman Empire and its first Emperor, ruling from 27 BC until his death in 14 AD.

Born into an old, wealthy equestrian branch of the plebeian Octavii family, in 44 BC Augustus was adopted posthumously by his maternal great-uncle Gaius Julius Caesar following Caesar's assassination. Together with Mark Antony and Marcus Lepidus, he formed the Second Triumvirate to defeat the assassins of Caesar. Following their victory at Phillipi, the Triumvirate divided the Roman Republic among themselves and ruled as military dictators. The Triumvirate was eventually torn apart under the competing ambitions of its members: Lepidus was driven into exile and stripped of his position, and Antony committed suicide following his defeat at the Battle of Actium by Augustus in 31 BC.

After the demise of the Second Triumvirate, Augustus restored the outward facade of the free Republic, with governmental power vested in the Roman Senate, the executive magistrates, and the legislative assemblies. In reality, however, he retained his autocratic power over the Republic as a military dictator. By law, Augustus held a collection of powers granted to him for life by the Senate, including supreme military command, and those of tribune and censor. It took several years for Augustus to develop the framework within which a formally republican state could be led under his sole rule. He rejected monarchical titles, and instead called himself Princeps Civitatis ("First Citizen"). The resulting constitutional framework became known as the Principate, the first phase of the Roman Empire.

The reign of Augustus initiated an era of relative peace known as the Pax Romana (The Roman Peace). Despite continuous wars or imperial expansion on the Empire's frontiers and one year-long civil war over the imperial succession, the Mediterranean world remained at peace for more than two centuries. Augustus dramatically enlarged the Empire, annexing Egypt, Dalmatia, Pannonia, Noricum, and Raetia, expanded possessions in Africa, expanded into Germania, and completed the conquest of Hispania.

Beyond the frontiers, he secured the Empire with a buffer region of client states, and made peace with the Parthian Empire through diplomacy. He reformed the Roman system of taxation, developed networks of roads with an official courier system, established a standing army, established the Praetorian Guard, created official police and fire-fighting services for Rome, and rebuilt much of the city during his reign.

Augustus died in 14 AD at the age of 75. He may have died from natural causes, although there were unconfirmed rumors that his wife Livia poisoned him. He was succeeded as Emperor by his adopted son (also stepson and former son-in-law), Tiberius.