Heraclius Constantine or Constantine III (Κωνσταντίνος Γ') (May 3, 612 - April 20/24 or May 26, 641) was the eldest son of the Byzantine emperor Heraclius and his first wife Eudocia, and ruled as Emperor for four months in 641.

Constantine was baptised and crowned on January 22, 613 and shortly after betrothed to his cousin, Gregoria, a daughter of his father's first cousin, Nicetas. As the couple were second cousins, the marriage was technically incestuous, but this consideration must have been outweighed by the advantages of the match to the family as a whole. Furthermore, its illegality paled into insignificance beside Heraclius' marriage to his niece Martina in the same year.

Constantine and Gregoria married in 629 or perhaps early 630 and in that year their first child, Constans II was born. Their second child was another son, Theodosius.

Constantine became emperor when his father died in 641 along with his younger half-brother Heracleonas, the son of Martina. His supporters feared action against him on the part of Martina and Heracleonas, and the treasurer Philagrius advised him to write to the army, informing them that he was dying and asking for their assistance in protecting the rights of his children. He also sent a vast sum of money, more than two million solidi (gold coins), to Valentinus Arshakouni, an adjutant of Philagrius, to distribute to the soldiers to persuade them to secure the succession for his sons after his death. Indeed, he died of tuberculosis after only four months, leaving Heracleonas sole emperor. A rumor that Martina had him poisoned led to deposition of the two of them in September that year and Constans II assumed the throne.

Preceded by: Heraclius

Byzantine Emperor

Succeeded by: Heracleonas