Tiberius II Constantine (Latin: Tiberius Cōnstantīnus; Ancient Greek: Τιβέριος Κωνσταντῖνος, romanized: Tibérios Kōnstantĩnos; died 14 August 582) was Eastern Roman emperor from 578 to 582. Tiberius rose to power in 574 when Justin II, during an abatement in a period of severe mental illness, proclaimed him caesar and adopted him as his own son. In 578, the dying Justin II gave him the title of augustus, thus making Tiberius co-emperor alongside him. Tiberius became sole ruler less than two weeks later, assuming the regnal name of "Constantine" under which he reigned until his death. Tiberius' reign was marked by warfare with the Persians, as well as Avar incursions into the empire.

Born in Thrace in the mid-6th century, of Greek descent, Tiberius was appointed to the post of notarius. There, sometime after 552, he was introduced by the Patriarch Eutychius to the future emperor, Justin II, with whom he became firm friends. Under Justin's patronage, Tiberius was promoted to the position of Comes excubitorum, which he held from approximately 565 through to 574. He was present during Justin's imperial accession on 14 November 565 and also attended his inauguration as consul on 1 January 566.

Justin ceased making payments to the Avars, which had been implemented by his predecessor, Justinian. In 569, he appointed Tiberius to the post of Magister utriusque militiae, with instructions to deal with the Avars and their demands. After a series of negotiations, Tiberius agreed to allow the Avars to settle on Roman territory in the Balkans, in exchange for male hostages taken from various Avar chiefs. Justin, however, rejected the agreement, insisting on taking hostages from the family of the Avar Khan himself. That condition was rejected by the Avars and so Tiberius mobilized for war.^{[citation needed]}

In 570, he defeated an Avar army in Thrace and returned to Constantinople. While attempting to follow up that victory in late 570 or early 571, Tiberius was defeated in a battle in the Danube regions in which he narrowly escaped death, as his army was fleeing the battlefield. Agreeing to a truce, Tiberius provided an escort to the Avar envoys to discuss the terms of a treaty with Justin. On their return, the Avar envoys were attacked and robbed by local tribesmen, prompting them to appeal to Tiberius for help. He tracked down the group responsible and returned the stolen goods.

In 574, Justin had a mental breakdown, forcing Empress Sophia to turn to Tiberius to manage the empire, which was fighting the Persians to the east and dealing with the internal crisis of the plague. To achieve a measure of breathing space, Tiberius and Sophia agreed to a one-year truce with the Persians, at the cost of 45,000 solidi. On 7 December 574, Justin, in one of his more lucid moments, had Tiberius proclaimed Caesar and adopted him as his own son. Tiberius added the name Constantine to his own. Although his position was now official, he was still subordinate to Justin. Sophia was determined to remain in power and kept Tiberius tightly controlled until Justin died, in 578.

The day after his appointment as Caesar, the plague abated, giving Tiberius more freedom of movement than Justin had been able to achieve. Tiberius also charted a very different course from his predecessor and proceeded to spend the money that Justin had saved in order to defend the imperial frontiers and win over the populace who had turned against him. According to Gregory of Tours, who was based in Merovingian Gaul, Tiberius found two treasures: the treasure of Narses and 1,000 centenaria: 100,000 pounds of gold or 7,200,000 solidi (nomismata), under a slab. The treasures were given away to the poor, to the consternation of Sophia. John of Ephesus, a contemporary East Roman, wrote that Tiberius lavishly gave presents to "all men" and later as emperor, gave money away to the rich rather than the poor. During his time as caesar, his spending was opposed by Justin and Sophia, who set a ceiling for his expenditure and restricted his access to the treasury.

Alongside generous donations, Tiberius also proceeded to reduce state revenue by removing taxes on wine and bread instituted by Justinian I and continued Justin's ban on the sale of governorships. In 575, he remitted tax arrears down to 571 and reduced taxes by a quarter for four years.

He also negotiated a truce with the Avars, paying them 80,000 solidi per year for which the Avars agreed to defend the Danube frontier, thereby allowing Tiberius to transfer troops across to the east for a planned renewal of the conflict against the Persians. In 575, Tiberius began moving the armies of Thrace and Illyricum to the eastern provinces. Buying time to make the necessary preparations, he agreed to an additional truce with the Persians for three years, paying 30,000 solidi annually, though the truce excluded the wars in Armenia. Not content with making preparations, Tiberius also used this period to send reinforcements to Italy under the command of Baduarius with orders to stem the Lombard invasion. He saved Rome from the Lombards and allied the Empire with Childebert II, the King of the Franks, to defeat them. Unfortunately, Baduarius was defeated and killed in 576, allowing even more imperial territory in Italy to slip away. Tiberius was unable to respond as the Sassanid Emperor Khosrau I struck at the empire's Armenian provinces in 576, sacking Melitene and Sebastea. Shifting his attention eastward, Tiberius sent his general Justinian with the eastern armies to push Khosrau and the Persians back across the Euphrates. The Byzantines led by Justinian pushed deep into Persian territory, culminating in a raid on Atropatene. In 577, however, Justinian was defeated in Persian Armenia, forcing a Byzantine withdrawal. In response to that defeat, Tiberius replaced Justinian with the future emperor Maurice. During the truce that Tiberius concluded with Khosrau, he busily enhanced the army of the east not only with transfers from his western armies but also through barbarian recruits, which he formed into a new foederati unit, amounting to some 15,000 troops by the end of his reign.