Byzantine emperor (from 565) whose attempts to maintain the integrity of the Byzantine Empire against the encroachments of the Avars, Persians, and Lombards were frustrated by disastrous military reverses.

A nephew and close adviser of the Byzantine emperor Justinian I, Justin II became emperor in November 565 following his uncle's death. He began his reign on a note of resolution and common sense; he paid state debts, remitted overdue taxes, and reduced expenditures.

In the early part of his reign, Justin allowed a measure of toleration to the dissident monophysite Christians. Initially, he hoped to bring about a union of the monophysite factions and then to unite them with the orthodox church. In March 571, however, he inaugurated a policy of persecution and issued a lengthy antimonophysitic creed that all clergy were required to sign under penalty of imprisonment.

In the West, despite an alliance with the Franks, Justin was unable to prevent the Lombards from entering Italy in 568, and parts of that country were soon permanently lost. His relations with the Avars (Avar) and Persians were marked by similar, though less serious, reverses. Shortly after his accession, determined to abandon Justinian's policy of buying peace, he rejected an Avar request for tribute. In 568 he concluded an alliance with the Western Turks of Central Asia, apparently directed against the Avars and Persians. Yet after campaigning against the Avars, who were ravaging the Danubian frontier, he was forced to come to terms with them in 571. Three years later a treaty was concluded stipulating that the Byzantines pay a yearly tribute to the Avars. In 576 the Western Turks, angered by the treaty, not only broke off their alliance with Justin but also seized a Byzantine stronghold in the Crimea.

In 571 the part of Armenia governed by Persia revolted and requested assistance from the Byzantine Empire. In the late summer of the following year, Justin's forces invaded Persia. The Persians, however, not only repulsed the Byzantines but themselves invaded Byzantine territory, capturing a number of important cities, including Dara, which fell in November 573. After learning of the fall of Dara, Justin became insane, and in 574 the empress Sophia, acting on his behalf, entered into peace negotiations.

Induced by Sophia to adopt as his son the general Tiberius, Justin conferred on him the title of caesar in December 574. Thereafter, Justin, although nominally still emperor, lived in retirement until his death.