The Byzantine navy was the naval force of the Byzantine Empire. Like the state it served, it was a direct continuation from its Roman predecessor, but played a far greater role in the defence and survival of the state than its earlier iteration. While the fleets of the Roman Empire faced few great naval threats, operating as a policing force vastly inferior in power and prestige to the army, command of the sea became vital to the very existence of the Byzantine state, which several historians have called a "maritime empire".

The first threat to Roman hegemony in the Mediterranean Sea was posed by the Vandals in the 5th century, but their threat was ended by the wars of Justinian I in the 6th century. The re-establishment of a permanently maintained fleet and the introduction of the dromon galley in the same period also marks the point when the Byzantine navy began departing from its late Roman roots and developing its own characteristic identity. This process would be furthered with the onset of the early Muslim conquests in the 7th century. Following the loss of the Levant and later Africa, the Mediterranean was transformed from a "Roman lake" into a battleground between the Byzantines and a series of Muslim states. In this struggle, the Byzantine fleets were critical, not only for the defence of the Empire's far-flung possessions around the Mediterranean basin, but also for repelling seaborne attacks against the imperial capital of Constantinople itself. Through the use of the newly invented "Greek fire", the Byzantine navy's best-known and feared secret weapon, Constantinople was saved from several sieges and numerous naval engagements resulted in Byzantine victories.

Initially, the defence of the Byzantine coasts and the approaches to Constantinople was borne by the great fleet of the Karabisianoi. Progressively however it was split up into several regional fleets, while a central Imperial Fleet was maintained at Constantinople, guarding the city and forming the core of naval expeditions. By the late 8th century, the Byzantine navy, a well-organized and maintained force, was again the dominant maritime power in the Mediterranean. Conflicts with navies of the Muslim world continued with alternating success, but in the 10th century, the Byzantines were able to gain supremacy in the Eastern Mediterranean.

During the 11th century, the navy, like the Empire itself, began to decline. Faced with new naval challenges from the West, the Byzantines were increasingly forced to rely on the navies of Italian city-states such as Venice and Genoa, with disastrous effects on the Empire's economy and sovereignty. A period of recovery under the Komnenos dynasty was followed by another period of decline, which culminated in the disastrous dissolution of the Empire by the Fourth Crusade in 1204. After the Empire was restored in 1261, several emperors of the Palaiologan dynasty attempted to revive the navy, but their efforts only had a temporary effect. Emperor Andronikos II Palaiologos even dissolved the navy completely, allowing Venice to defeat the Byzantines in two wars, the first of which resulted in a humiliating treaty that saw the Venetians keep multiple islands captured from Byzantine forces during the war and forced the latter to repay Venice for the destruction of Constantinople's Venetian quarter at the hands of the city's Genoese residents. By the mid-14th century, the Byzantine fleet, which once could put hundreds of warships to sea, was limited to a few dozen at best, and control of the Aegean Sea definitively passed to Italian navies and, in the 15th century, the nascent Ottoman Navy. The diminished Byzantine navy continued to be active until the fall of Constantinople to the Ottoman Empire in 1453.

The Byzantine navy, like the Eastern Roman or Byzantine Empire itself, continued the systems of the Roman Empire. After the Battle of Actium in 31 BC, in the absence of any external threat in the Mediterranean, the Roman navy performed mostly policing and escort duties. Massive sea battles, like those fought centuries before in the Punic Wars (264 to 146 BC), no longer occurred, and the Roman fleets comprised relatively small vessels, best suited to their new tasks. By the early 4th century AD, the permanent Roman fleets had dwindled, so that when the fleets of the rival emperors Constantine the Great and Licinius clashed in 324 AD, they were composed to a great extent of newly built or -commandeered ships from the port cities of the Eastern Mediterranean. The civil wars of the 4th and early 5th centuries, however, did spur a revival of naval activity, with fleets mostly employed to transport armies. Considerable naval forces continued to be employed in the Western Mediterranean throughout the first quarter of the fifth century, especially from North Africa, but Rome's mastery of the Mediterranean was challenged when Africa was overrun by the Vandals (429 to 442).

The new Vandalic Kingdom of Carthage, under the capable Geiseric (r. 428–477), immediately launched raids against the coasts of Italy and Greece, even sacking and plundering Rome in 455. The Vandal raids continued unabated over the next two decades, despite repeated Roman attempts to defeat them. The Western Empire was impotent, its navy having dwindled to almost nothing, but the eastern emperors could still call upon the resources and naval expertise of the eastern Mediterranean. A first Eastern expedition in 448, however, went no further than Sicily, and in 460, the Vandals attacked and destroyed a Western Roman invasion fleet at Cartagena in Spain. Finally, in 468, a huge Eastern expedition assembled under Basiliscus, reputedly numbering 1,113 ships and 100,000 men, but it failed disastrously. About 600 ships were lost to fire ships, and the financial cost of 130,000 pounds of gold and 700,000 pounds of silver nearly bankrupted the Empire. This forced the Romans to come to terms with Geiseric and to sign a peace treaty. After Geiseric's death in 477, however, the Vandal threat receded.

The 6th century marked the rebirth of Roman naval power. In 508, as antagonism with the Ostrogothic Kingdom of Theodoric flared up, the Emperor Anastasius I (r. 491–518) is reported to have sent a fleet of 100 warships to raid the coasts of Italy. In 513, the general Vitalian revolted against Anastasius. The rebels assembled a fleet of 200 ships which, despite some initial successes, were destroyed by admiral Marinus, who employed a sulphur-based incendiary substance to defeat them.

In 533, taking advantage of the absence of the Vandal fleet, sent to suppress a revolt in Sardinia, an army of 15,000 under Belisarius was transported to Africa by an invasion fleet of 92 dromons and 500 transports, beginning the Vandalic War, the first of the wars of the reconquest of Emperor Justinian I (r. 527–565). These were largely amphibious operations, made possible by the control of the Mediterranean waterways, and the fleet played a vital role in carrying supplies and reinforcements to the widely dispersed Byzantine expeditionary forces and garrisons. This fact was not lost on the Byzantines' enemies. Already in the 520s, Theodoric had planned to build a massive fleet directed against the Byzantines and the Vandals, but his death in 526 limited the extent to which these plans were realized. In 535, the Gothic War began with a double-pronged Byzantine offensive, with a fleet again carrying Belisarius' army to Sicily and then Italy, and another army invading Dalmatia. Byzantine control of the sea was of great strategic importance, allowing the smaller Byzantine army to successfully occupy the peninsula by 540.