Byzantium under the Heraclians refers to the period when the East Roman or Byzantine Empire is ruled by the emperors of the Heraclian dynasty.

The Heraclians preside over a period of cataclysmic events that are a watershed in the history of the Empire and the world in general.At the beginning of the dynasty, the Empire is still recognizable as the Eastern Roman Empire, dominating the Mediterranean and harboring a prosperous Late Antiquity urban civilization.

This world is shattered by successive invasions, which result in extensive territorial losses, financial collapse and plagues that depopulate the cities, while religious controversies and rebellions further weaken the Empire.By the dynasty's end, a very different state has emerged: medieval Byzantium, a chiefly agrarian, military-dominated society that is engaged in a lengthy struggle with the Muslim Caliphate.

However, this state is also far more homogeneous, being reduced to its mostly Greek-speaking and firmly Chalcedonian core territories, which enable it to weather these storms and enter a period of stability under the successor Isaurian Dynasty.The Heraclian dynasty is named after the general Heraclius the Younger, who in 610 sailed from Carthage, overthrows the usurper Phocas and is crowned Emperor.

At this time, the Empire is embroiled in a war with the Sassanid Persian Empire, which in the next decade conquers the Empire's eastern provinces.After a long and exhausting struggle, Heraclius manages to defeat the Persians and restore the Empire, only to lose these provinces again shortly after to the sudden eruption of the Muslim conquests.

His successors struggle to contain the Arab tide.

The Levant and North Africa are lost, while in 674–678, a large Arab army besieges Constantinople itself.Nevertheless, the state survives and the establishment of the Theme system allows the imperial heartland of Asia Minor to be retained.

Under Justinian II and Tiberios III the imperial frontier in the East is stabilized, although incursions continue on both sides.The latter 7th century also sees the first conflicts with the Bulgars and the establishment of a Bulgarian state in formerly Byzantine lands south of the Danube, which will be the Empire's chief antagonist in the West until the 11th century.