The Battle of Philippi was the final battle in the Wars of the Second Triumvirate between the forces of Mark Antony and Octavian (of the Second Triumvirate) and the leaders of Julius Caesar's assassination, Brutus and Cassius in 42 BC, at Philippi in Macedonia. The Second Triumvirate declared the civil war ostensibly to avenge Julius Caesar's assassination in 44 BC, but the underlying cause was a long-brewing conflict between the so-called Optimates and the so-called Populares.

The battle, involving up to 200,000 men in one of the largest of the Roman civil wars, consisted of two engagements in the plain west of the ancient city of Philippi. The first occurred in the first week of October; Brutus faced Octavian, and Antony's forces fought those of Cassius. The Roman armies fought poorly, with low discipline, nonexistent tactical coordination and amateurish lack of command experience evident in abundance with neither side able to exploit opportunities as they developed. At first, Brutus pushed back Octavian and entered his legions' camp. However, to the south, Cassius was defeated by Antony and committed suicide after hearing a false report that Brutus had also failed. Brutus rallied Cassius's remaining troops, and both sides ordered their army to retreat to their camps with their spoils. The battle was essentially a draw but for Cassius's suicide. A second encounter, on 23 October, finished off Brutus's forces after a hard-fought battle. He committed suicide in turn, leaving the triumvirate in control of the Roman Republic.