Yazdegerd III or Yazdgerd III (also spelled Yazdiger or Yazdigerd, Persian: یزدگرد, "made by God") was the twenty-ninth and last king of the Sassanid dynasty of Iran and a grandson of Khosrau II (590–628). His father was Shahryar, whose mother was Miriam, the daughter of the Byzantine Emperor Maurice. Yazdegerd III ascended the throne on 16 June 632 after a series of internal conflicts.

Yazdegerd was born in central Iran, reigned as a youth and had never truly exercised authority. The Muslim conquest of Persia began in his first year of reign, and ended with the Battle of al-Qādisiyyah. Yazdegerd sought an alliance with Emperor Heraclius, who was an old rival of the Persian Empire.

Following the battle of al-Qādisiyyah, the Arabs occupied Ctesiphon, and the young king fled eastward into Media going from one district to another, until at last he was killed by a local miller for his purse at Merv in 651.

The legend is that he was killed by a miller who robbed him of his clothes and jewellery, but there is a strong suspicion that the governor of Merv, was the real culprit.