Tiberius was a Germanic naval officer from the region of Pamphylia and originally named Apsimar (Αψίμαρος, Apsímaros), who rose to the position of droungarios of the Cibyrrhaeotic Theme. He participated in the failed campaign to regain Carthage in 698. As admiral John the Patrician retreated from Carthage to Crete, the fleet rebelled, deposed and murdered their commander, and chose Apsimaros as his replacement. Changing his name to Tiberius, Apsimaros sailed on Constantinople which was suffering from a plague and proceeded to besiege it.
His revolution attracted the support of the Green faction, as well as detachments from the field army and the imperial guard, and officers loyal to him opened the gates of the city and proclaimed him emperor, after which his troops then proceeded to pillage the city. When he was firmly established on the throne, he commanded that the nose of deposed Emperor Leontius be cut off, and ordered him to enter the monastery of Psamathion. Leontios had also mutilated his predecessor Justinian II in the same fashion three years earlier.
As emperor, Tiberius III made the tactical decision to ignore Africa, where Carthage was now definitively lost. Instead, he appointed his brother Heraclius as monostrategos of the East, who firstly strengthened the land and sea defences of Anatolia before proceeding to attack the Umayyad Caliphate under Abd al-Malik, winning minor victories while raiding into northern Syria in 700 and 701. He then proceeded to invade and for a period hold territory in Armenia, while Arab reprisals in 703 and 704 were repelled from Cilicia with heavy Arab losses.
Success in the military sphere was accompanied by Tiberius's attempt to strengthen the empire militarily by reorganizing its administration. Tiberius then turned his attention to the Island of Cyprus, which had been underpopulated since the reign of Justinian II. He sent a delegation to the Caliph at Damascus, asking for the return of many Cypriot prisoners who had been captured near the Propontis, and subsequently returned them to their place of birth. He strengthened the defence of the island at the same time by increasing the garrison numbers with troops from the Taurus Mountains. He also reorganized the Cibyrrhaeotic Theme and repaired the sea walls of Constantinople.
Domestically, his only known act of note was the banishment of Philippikos Bardanes, the son of a notable patrician, to the island of Cephalonia. Philippikos, a future emperor, had dreamed that his head was overshadowed by an eagle, which Tiberius took to mean that he was planning to rebel against him.