The Battle of Actium was a naval battle fought between a maritime fleet of Octavian led by Marcus Agrippa and the combined fleets of both Mark Antony and Cleopatra VII Philopator. The battle took place on 2 September 31 BC in the Ionian Sea, near the former Roman colony of Actium, Greece, and was the climax of over a decade of rivalry between Octavian and Antony.

In early 31 BC, the year of the battle, Antony and Cleopatra were temporarily stationed in Greece. Mark Antony possessed 500 ships and 70,000 infantry, and made his camp at Actium, and Octavian, with 400 ships and 80,000 infantry, arrived from the north and occupied Patrae and Corinth, where he managed to cut Antonys southward communications with Egypt (via the Peloponnese) with help from Marcus Agrippa. Octavian previously gained a preliminary victory in Greece, where his navy successfully ferried troops across the Adriatic Sea under the command of Marcus Agrippa. Octavian landed on mainland Greece, opposite of the island of Corcyra (modern Corfu) and proceeded south, on land.

Trapped on both land and sea, portions of Antony's army deserted and fled to Octavian's side (daily), and Octavian's forces became comfortable enough to make preparations for battle. Antony's fleet sailed through the bay of Actium on the western coast of Greece, in a desperate attempt to break free of the naval blockade. It was there that Antony's fleet faced the much larger fleet of smaller, more manoeuvrable ships under commanders Gaius Sosius and Agrippa. Antony and his remaining forces were spared only due to a last-ditch effort by Cleopatra's fleet that had been waiting nearby. Octavian pursued them and defeated their forces in Alexandria on 1 August 30 BCafter which Antony and Cleopatra committed suicide.

Octavian's victory enabled him to consolidate his power over Rome and its dominions. He adopted the title of Princeps ("first citizen"), and in 27 BC was awarded the title of Augustus ("revered") by the Roman Senate. This became the name by which he was known in later times. As Augustus, he retained the trappings of a restored Republican leader, but historians generally view his consolidation of power and the adoption of these honorifics as the end of the Roman Republic and the beginning of the Roman Empire.

The War of Actium (32–30 BC) was the last civil war of the Roman Republic, fought between Mark Antony (assisted by Cleopatra) and Octavian. In 32 BC, Octavian convinced the Roman Senate to declare war on the Egyptian queen Cleopatra. Her lover and ally Mark Antony, who was Octavian's rival, gave his support for her cause. Forty-percent of the Roman Senate, together with both consuls, left Rome to join the war on Antony's side. After a decisive victory for Octavian at the Battle of Actium, Cleopatra and Antony withdrew to Alexandria, where Octavian besieged the city until both Antony and Cleopatra were forced to commit suicide.

The war involved some of the largest Roman armies ever seen. Both Antony and Octavian's legions were experienced veterans of previous civil wars who had fought together, many also having once served under Caesar. The two did however raise their own legions separately.

Following the end of the war, Octavian brought peace to the Roman state that had been plagued by a century of civil wars. Octavian became the most powerful man in the Roman world and the Senate bestowed upon him the honorific of Augustus in 27 BC. Octavian, now Augustus, would be the first Roman emperor and would transform the Republic into the Roman Empire.

The last Republican civil war would mark the beginning of the Pax Romana, a period of relative internal peace and stability.