The Gothic War between the Byzantine Empire during the reign of Emperor Justinian I and the Ostrogothic Kingdom of Italy took place from 535 to 554 in the Italian peninsula, Dalmatia, Sardinia, Sicily, and Corsica. It was one of the last of the many Gothic wars against the Roman Empire. The war had its roots in the ambition of the Byzantine emperor Justinian I to recover the provinces of the former Western Roman Empire, which the Romans had lost to invading barbarian tribes in the previous century, during the Migration Period.

The war followed the Roman reconquest of the diocese of Africa from the Vandals. Historians commonly divide the war into two phases. The first phase lasts from 535 to the fall of the Ostrogothic capital Ravenna in 540, and the apparent reconquest of Italy by the Byzantines. The second phase from 540/541 to 553 featured a Gothic revival under Totila, which was suppressed only after a long struggle by the Roman general Narses, who also repelled an invasion in 554 by the Franks and Alamanni.

In 554, Justinian promulgated a pragmatic sanction that prescribed Italy's new government. Several cities in northern Italy held out against Constantinople until 562. By the end of the war, Italy had been devastated and depopulated. It was seen as a pyrrhic victory for the Eastern Romans, who found themselves incapable of resisting an invasion by the Lombards in 568, which resulted in Constantinople permanently losing control over large parts of the Italian peninsula.

In 476, Odoacer deposed Emperor Romulus Augustulus and declared himself King of Italy (rex Italiae), resulting in the final dissolution of the Western Roman Empire in Italy. Although Odoacer recognised the nominal suzerainty of the Byzantine emperor, Zeno, his independent policies and increasing strength made him a threat in the eyes of Constantinople. To provide a buffer, the Ostrogoths, under their leader, Theodoric the Great, were settled as foederati (allies) of the empire in the western Balkans, but unrest continued. Zeno sent the Ostrogoths to Italy as the representatives of the empire to remove Odoacer. Theodoric and the Goths defeated Odoacer, and Italy came under Gothic rule. In the arrangement between Theodoric and Zeno and his successor Anastasius, the land and its people were regarded as part of the Empire, with Theodoric a viceroy and head of the army (magister militum). This arrangement was scrupulously observed by Theodoric; there was continuity in civil administration, which was staffed exclusively by Romans, and legislation remained the preserve of the emperor. The army, on the other hand, was exclusively Gothic, under the authority of their chiefs and courts. The peoples were also divided by religion: the Romans were Chalcedonian Christians, while the Goths were Arian Christians. Unlike the Vandals or the early Visigoths, the Goths practised considerable religious tolerance. The dual system worked under the capable leadership of Theodoric, who conciliated the Italian aristocracy, but the system began to break down during his later years and collapsed under his heirs.

With the ascension of Emperor Justin I, the end of the Acacian schism between the Eastern and Western churches, and the return of ecclesiastical unity within the East, several members of the Italian senatorial aristocracy began to favour closer ties to Constantinople to balance Gothic power. The deposition and execution of the distinguished magister officiorum Boethius and his father-in-law in 524 were part of the slow estrangement from the Gothic regime. Theodoric was succeeded by his 10-year-old grandson Athalaric in August 526, with his mother, Amalasuntha, as regent; she had received a Roman education and began a rapprochement with the Senate and the Empire. This conciliation and Athalaric's Roman education displeased Gothic magnates, who plotted against her. Amalasuntha had three of the leading conspirators killed and wrote to Justinian, asking for sanctuary if she was deposed. Amalasuntha remained in Italy.

In 533, using a dynastic dispute as a pretext, Justinian sent his most talented general, Belisarius, to recover the North African provinces held by the Vandals. The Vandalic War produced an unexpectedly swift and decisive victory for the Byzantine Empire and encouraged Justinian in his ambition to recover the rest of the lost western provinces. As regent, Amalasuntha had allowed the Byzantine fleet to use the harbours of Sicily, which belonged to the Ostrogothic Kingdom. After the death of Athalric in 534, Amalasuntha offered the kingship to her cousin Theodahad, who accepted the offer but then had her arrested; she was killed in early 535. Justinian made unsuccessful attempts through his agents to save Amalasuntha's life, and her death gave him a casus belli to go to war with the Goths. Procopius wrote that "as soon as he learned what had happened to Amalasuntha, being in the ninth year of his reign, he entered into war".

Belisarius was appointed commander in chief (strategos autokrator) for the expedition against Italy with 7,500 men. Mundus, the magister militum per Illyricum, was ordered to occupy the Gothic province of Dalmatia. The forces made available to Belisarius were small, especially when compared with the much larger army he had fielded against the Vandals, an enemy much weaker than the Ostrogoths. The preparations for the operation were carried out in secret, while Justinian tried to secure the neutrality of the Franks with gifts of gold.

Belisarius landed at Sicily, between Roman Africa and Italy, whose population was well disposed toward the Empire. The island was quickly captured, with the only determined resistance, at Palermo, overcome by late December. Belisarius prepared to cross to Italy and Theodahad sent envoys to Justinian, proposing at first to cede Sicily and recognise his overlordship but later to cede all of Italy.