Yazdegerd I, or Izdekerti ("made by God"), is the thirteenth Sassanid king of Persia and rules from 399 to 421.

He is believed by some to be the son of Shapur III (383–388).

He succeeds to the Persian throne on the assassination of Bahram IV in 399 and rules for twenty-one years until his death in 421.

Yazdegerd I's reign is largely uneventful.

The king is described as being of a peaceful disposition.

There are cordial relations between Persia and the Eastern Roman Empire as well as between Persia and the Western Roman Empire.

Early during his reign, Yazdegerd is entrusted with the care of the Roman prince Theodosius by his father Arcadius on the latter's death in 408, and Yazdegerd faithfully defends the life, power and possessions of the Roman prince.

Yazdegerd promotes Christianity in the early years of his reign and later opposes it.

His alternate persecution of Zoroastrians and later Christians earns him the epithets of Al Khashaor "the Harsh" and Al Athim or "the Wicked" and Yazdegerd the Sinner.

However, his general disposition towards the citizens of the Persian Empire is good.

They give him the epithet of Ramashtras or "the most quiet".

The later part of his reign is occupied by his attempts to convert Armenia to Zoroastrianism.

During his last days, there occurs a civil war between his sons.

Bahram V emerges victorious and claims the throne.

Yazdegerd I dies in 421 and is succeeded by his son Bahram V or Bahramgur.