Yazdegerd I ("made by God" Izdigerdes), king of Persia, son of Shapur III, 399-420, called "the sinner" by the Persians.

He was a highly intelligent ruler, who tried to emancipate himself from the dominion of the magnates and the Magian priests. He punished the nobles severely when they attempted oppression; he stopped the persecution of the Christians and granted them their own organization.

With the Roman Empire he lived in peace and friendship, and is therefore as much praised by the Byzantine authors (Procop. Pers. i. 2; Agath. iv. 26) as he is blamed by the Persians.

After a reign of twenty years he appears to have been murdered in Khorasan.