The siege of Brundisium was an early military confrontation of Caesar's Civil War. Taking place in March 49 BC, it saw the forces of Gaius Julius Caesar's Populares besiege the Italian city of Brundisium on the coast of the Adriatic Sea which was held by a force of Optimates under the command of Gnaeus Pompeius Magnus. After a series of brief skirmishes, during which Caesar tried to blockade the harbour, Pompey abandoned the city and managed to evacuate his men across the Adriatic to Epirus. Pompey's retreat meant that Caesar had full control over the Italian Peninsula, with no way to pursue Pompey's forces in the east he instead decided to head west to confront the legions Pompey had stationed in Hispania.

Escalating tensions over the previous decade between Gaius Julius Caesar and the Roman Senate, rallying around Gnaeus Pompeius Magnus ("Pompey the Great"), culminated in Caesar crossing the Rubicon river in January 49 BC and thus being labelled as an enemy of the people. Caesar rapidly advanced down the Italian Peninsula capturing numerous cities, towns and settlements such as Arretium, Auximum and Asculum. Most significant was the siege of Corfinium where Caesar's bitter enemy Lucius Domitius Ahenobarbus was handed over to him in a bloodless victory which was achieved in only one week. Ahenobarbus had disobeyed Pompey's orders to rendezvous with him in Apulia where he was amassing his own forces, and as a result 33 of Pompey's cohorts surrendered to Caesar at Corfinium.

Upon hearing the news of the defeat at Corfinium Pompey marched his men from his base in Luceria to Canusium, and then on to Brundisium. Once at Brundisium he again put out the order for all available men to hasten to him, he also set about arming nearby slaves and through this practice managed to levy 300 horsemen.

Caesar had immediately broken camp once the siege of Corfinium was completed and headed south into Apulia to pursue Pompey, passing through the lands of the Marrucini, the Frentani, and the Larinati. The praetor Lucius Manlius Torquatus managed to flee Alba Fucens and reach Pompey with 6 cohorts, while another praetor Lucius Rutilius Lupus was intercepted leaving Tarracina with 3 cohorts by Caesar's cavalry commanded by Bivius Curius. During his march south Caesar managed to recruit several cohorts to his cause. Pompey's chief-engineer, Cnaeus Magius, was captured and sent along to Brundisium with the message that Caesar wished for an audience with Pompey where the two could more easily settle their differences than via messengers.

Caesar arrived at Brundisium on 9 March 49 BC with a total of 3 veteran legions (Legio VIII, Legio XII and Legio XIII) along with 3 freshly levied legions. Pompey remained within the city with the equivalent of 2 legions. He had been overseeing a massive logistical operation which involved requisitioning merchant vessels so that the vast majority of his forces, and most of the prominent senators who had accompanied him, had already been transferred across the Adriatic to Epirus in Greece. Unsure of whether Pompey intended to stay at Brundisium and retain a foothold in Italy, from which to better exert control over both the peninsula and the Adriatic, or whether he was simply waiting for the remainder of his men to be evacuated Caesar resolved to besiege the city and cut off Pompey's communication lines in the event that the former was true. His plan of action was to build two mounds of dirt on each side of the harbour outside the city, then across the expanse of water would be anchored a number of double-floats. These floats would be thirty feet wide and covered with dirt so as to make them more easily defensible. On every fourth float there was to be constructed a two-storey high defensive turret as well as at either end of the structure.

In response to this Pompey gathered boats from the harbour, erected his own three-storey high turrets upon them, and equipped them with siege engines so that he might be able to harry Caesar's defences. To this end a number of skirmishes occurred between the opposing sides, mainly using missiles, as Pompey's men tried to break through the floats and disrupt the construction work.

Still believing there was a chance that Pompey would step back and hold peace negotiations, Caesar claims he approached hostilities in a solely defensive manner. He had been surprised that Pompey had not answered his previous request for a meeting, but nonetheless sent Gaius Caninius Rebilus to meet with one of Pompey's men named Lucius Scribonius Libo. Rebilus' task was to convince Libo, who was a close personal friend of his, to arrange a meeting between Caesar and Pompey where a peace could be negotiated. The answer that Pompey gave Libo was that any such negotiations would be impossible as neither of the serving consuls were present to oversee them. Caesar took this to mean that the opportunity for a peaceful settlement at Brundisium was over and so he began more aggressive action.

After nine days, and at around the time where Caesar's flotilla was halfway complete, Pompey's transport fleet returned from Dyrrachium in Greece and passed through the blockade into Brundisium. Pompey began preparations for himself and the rest of his men to leave the city; he ordered his men to harass Caesar's forces and began reinforcing the streets and gates of the city with trenches, stakes, and palisades so as to limit the chances of Caesar's men infiltrating the city at the moment of the evacuation. To this end he had all the gates to the city barred up and built two fortified passageways leading to the port. When the preparations were complete he ordered his men to quietly board the newly arrived ships under cover of these passageways, and posted some lightly armed sentries on the walls to further cover his true intentions. Once his men were all embarked, Pompey was to signal to this small detachment on the walls who would then head for some rowboats which had been stored for them in a secure location.