Procopius' Wars of Justinian (Greek: Ὑπέρ τῶν πολέμων λόγοι, Latin: De Bellis, "About the Wars") is clearly his most important work, although it is not as well-known as the Secret History. The first seven books, which may have been published as a unit, seem to have been largely completed by 545, but were updated to mid-century before publication, for the latest event mentioned belongs to early 551. The first two books (often known as the Persian War, Latin De Bello Persico) deal with the conflict between the Romans and Sassanid Persia in Mesopotamia, Syria, Armenia, Lazica and Caucasian Iberia (roughly modern-day Georgia). It details the campaigns of the Sasanian Shah Kavadh I, the 'Nika' revolt in Constantinople in 532, the war by Kavadh's successor, Khosrau I, in 540 and his destruction of Antioch and the transportation of its inhabitants to Mesopotamia, and the great plague that devastated Constantinople in 542. They also cover the early career of the Roman general Belisarius, Procopius' patron, in some detail. The next two books, the Vandal War (Latin De Bello Vandalico), cover Belisarius' successful campaign against the Vandal kingdom in Roman Africa. The remaining books cover the Gothic War (Latin De Bello Gothico), the campaigns by Belisarius and others to recapture Italy, then under the rule of the Ostrogoths. This includes accounts of the sieges of Naples and Rome.

Later, Procopius added an eighth book (Wars VIII or Gothic War IV) which brings the history to 552/553, when a Roman army led by the eunuch Narses finally destroyed the Ostrogothic kingdom. This eighth book covers campaigns both in Italy and on the Eastern frontier.

The Wars of Justinian was influential on later Byzantine history-writing.[12] A continuation of Procopius' work was written after Procopius' death by the poet and historian Agathias of Myrina.