The 1st century BC, also known as the last century BC, started on the first day of 100 BC and ended on the last day of 1 BC. The AD/BC notation does not use a year zero; however, astronomical year numbering does use a minus sign, so "2 BC" is equal to "year -1". This is the 99th century in the Holocene calendar; it spans the years 9,901 to 10,000. 1st century AD (Anno Domini) follows.

In the course of the century all the remaining independent lands surrounding the Mediterranean were steadily brought under Roman control, being ruled either directly under governors or through puppet kings appointed by Rome. The Roman state itself was plunged into civil war several times, finally resulting in the marginalization of its 500 year old republic, and the embodiment of total state power in a single man—the emperor.

The internal turbulence that plagued Rome at this time can be seen as the last death throes of the Roman Republic, as it finally gave way to the autocratic ambitions of powerful men like Julius Caesar, Mark Antony and Octavian. Octavian's ascension to total power as the emperor Augustus is considered to mark the point in history where the Roman Republic ends and the Roman Empire begins. Some scholars refer to this event as the Roman Revolution. It is believed that the birth of Jesus, the central figure of Christianity took place at the close of this century.

The 1st century was the century that lasted from 1 to 100 according the Julian calendar. It is considered part of the Classical era, epoch, or historical period.

During this period Europe, North Africa and the Near East fell under increasing domination by the Roman Empire, which continued expanding, most notably conquering Britain under the emperor Claudius (43). The reforms introduced by Augustus during his long reign stabilized the empire after the turmoil of the previous century's civil wars. Later in the century the Julio-Claudian dynasty, which had been founded by Augustus came to an end with the death of Nero in 68. There followed the famous Year of Four Emperors, a brief period of civil war and instability, which was finally brought to an end by Vespasian, 9th Roman emperor, and founder of the Flavian dynasty.

China continued to be dominated by the Han Dynasty, despite a 14-year interruption by the Xin dynasty under Wang Mang. Han rule was restored in 23; Wang Mang's rule represents the watershed between the Western/Former Han and the Eastern/Later Han. The capital was also moved from Chang'an to Luoyang.