Heraclius (Greek: Ἡράκλειος Herakleios; c. 575 – 11 February 641), sometimes called Heraclius I, was the Byzantine emperor from 610 to 641. His rise to power began in 608, when he and his father, Heraclius the Elder, the exarch of Africa, led a revolt against the unpopular usurper Phocas.

Heraclius's reign was marked by several military campaigns. The year Heraclius came to power, the empire was threatened on multiple frontiers. Heraclius immediately took charge of the Byzantine–Sasanian War of 602–628. The first battles of the campaign ended in defeat for the Byzantines; the Persian army fought their way to the Bosphorus but Constantinople was protected by impenetrable walls and a strong navy, and Heraclius was able to avoid total defeat. Soon after, he initiated reforms to rebuild and strengthen the military. Heraclius drove the Persians out of Asia Minor and pushed deep into their territory, defeating them decisively in 627 at the Battle of Nineveh. The Persian king Khosrow II was overthrown and executed by his son Kavad II, who soon sued for a peace treaty, agreeing to withdraw from all occupied territory. This way peaceful relations were restored to the two deeply strained empires.

However, Heraclius soon lost many of his newly regained lands to the Rashidun Caliphate. Emerging from the Arabian Peninsula, the Muslims quickly conquered the Sasanian Empire. In 636, the Muslims marched into Roman Syria, defeating Heraclius's brother Theodore. Within a short period of time, the Arabs conquered Mesopotamia, Armenia and Egypt. Heraclius responded with reforms which allowed his successors to combat the Arabs and avoid total destruction.

Heraclius entered diplomatic relations with the Croats and Serbs in the Balkans. He tried to repair the schism in the Christian church in regard to the Monophysites, by promoting a compromise doctrine called Monothelitism. The Church of the East (commonly called Nestorian) was also involved in the process. Eventually this project of unity was rejected by all sides of the dispute.