Another son, Bahram V, hurried to the Sasanian capital of Ctesiphon with an Arab army and pressured the nobility to acknowledge him as shah.

It is known in other languages as Yazdekert (Pahlavi); Yazd[e]gerd (New Persian); Yazdegerd, Izdegerd and Yazdeger (Syriac); Yazkert (Armenian); Izdeger and Azger (in the Talmud); Yazdeijerd (Arabic), and Isdigerdes (Greek).

[2] Yazdegerd I inherited an empire which had been through tumultuous times; his three previous predecessors, Bahram IV, Shapur III and Ardashir II, had been murdered by the nobility.

[1] The Roman emperor Arcadius (r. 383–408) asked Yazdegerd for aid to guarantee the succession of his young son, Theodosius, as a result of the shah's generosity.

[8][1] According to Procopius, "Loyally observing the behests of Arcadius, [Yazdegerd] adopted and continued without interruption a policy of profound peace with the Romans, and thus preserved the empire for Theodosius.

[13] According to the modern historian Eberhard Sauer, Sasanian shahs persecuted other religions only when it was in their urgent political interest to do so.

[15] They were near the court in the Sasanian capital of Ctesiphon, indicating the consent of Yazdegerd (who financed churches with East Syrian or Roman diplomats as their main patrons).

[15] Although priestly leaders such as Shemon Bar Sabbae and his colleagues had zealously opposed Shapur II's request to participate in the imperial bureaucracy, the bishops began operating as agents of Iran (dissociating themselves from Zoroastrianism) during the fifth century.

[15] Yazdegerd made use of the priestly leaders, sending the Patriarch of the Catholicos of Ctesiphon to mediate between himself and his brother (the governor of Pars, in southern Iran).

[19] Owing to his tolerant treatment of the Christians, he is described in their chronicles as a "noble soul" and a second Cyrus the Great (r. 550 – 530 BC), the founder of the Iranian Achaemenid Empire.

[21] Abda, the bishop of Ohrmazd-Ardashir in Khuzestan, and a band of Christian priests and laity levelled a Zoroastrian fire temple in c. 419–420; the court summoned them to answer for their actions.

[22] Yazdegerd was said to ask Abda, "Since you are the chief and leader of these men, why do you allow them to despise our kingdom, to transgress against our command, and to act in accordance with their own will?

[14] Probably due to his change of policy, Yazdegerd appointed Mihr Narseh of the Suren family as his minister (wuzurg framadar).

[27] The Middle Persian geography text Šahrestānīhā ī Ērānšahr (The Provincial Capitals of Iran) reports that Yazdegerd had Jews settled in Spahan at Shushandukht's request,[27] and she was the mother of his son Bahram V.[27] According to the Iranologist Ernst Herzfeld, the Tomb of Esther and Mordechai in Hamadan was not the burial site of Esther and Mordechai but that of Shushandukht.

[29] This hostile view of Yazdegerd is due to his peaceful attitude towards the Romans and his religious tolerance of the country's non-Zoroastrians (the Christians and Jews).

[31] Yazdegerd I's coins portray him wearing a combination of the dome-shaped crown used by Ardashir II and two merlons and a crescent moon on the top.

[1] His reign marks a shift in the political perspective of the Sasanian Empire, which (originally disposed towards the West) moved to the East.

[33][34][b] In the Middle Persian heroic poem Ayadgar-i Zariran (The Testament of Zarer), the title was used by the last Kayanian monarch (Vishtaspa) and occurs in the 10th-century Zoroastrian Denkard.

[40] Yazdegerd I is notable for having ordered the renewal of a number cities, which include Qumis, Hamadan, Susa, Shushtar, and Spahan.