Yezdegerd, Or Isdigerd, the name of three Persian kings of the Sassanian dynasty. - Yezdegerd I. reigned from A. D. 399 to about 420. He maintained peace abroad, although the condition of the empire was favorable to the renewal of hostilities with Rome. He alternately persecuted the Magians and the Christians, according to changes in his own views, and few sovereigns have been more generally execrated. - Yezdegerd II. reigned from about 440 to 457. He declared war against the Romans and invaded their territory, but accepted proposals for peace from Theodosius II. After a nine years' war with the Ephthalites, a Tartar tribe on his N. frontier, he expelled their monarch. He compelled the Armenians by force of arms to accept Zoroastrianism; their patriarch Joseph and other bishops were martyred. In an expedition against the insurgent Ephthalites Yezdegerd was drawn into an ambush and defeated, and soon after died. - Yezdegerd III., born about 617, reigned from 632 to 651. He at once had to contend with the Moslem invaders. In 636 was fought the battle of Cadesia (Kadisiyeh), lasting four days, in which the Persians were defeated and their renowned general Rustam was slain'. In 637 Ctesiphon, the Persian capital, fell into the hands of the Arabs, and at Jalula soon after the Mohammedans were again victorious.

In 641 Yezdegerd collected at Nehavend an army of 150,000 men, but, outwitted by Noman, a general of the caliph Omar, was defeated by a much inferior force. This battle overthrew the Sassanian power. Yezdegerd lived ten years a fugitive, and was at last slain by one of his own former subjects. (See Sassanid^e, and Persia).