Who was the last Zoroastrian monarch of the Sassanid Empire?

The last Zoroastrian monarch of the Sassanid Empire was Yazdegerd III.

He ruled as the final Sasanian King of Kings from 632 until 651, during the period when the weakened empire faced the Muslim conquest of Persia. He inherited a realm already damaged by dynastic violence, civil conflict, plague, and prolonged war with Byzantium, and he was only about eight years old when enthroned.

Yazdegerd never fully restored centralized authority. Powerful military commanders and aristocratic factions controlled much of the government, while the shah moved from region to region seeking support. After defeat, he fled eastward and was killed near Merv in 651. His death ended the Sasanian dynasty and the last major pre-Islamic Iranian empire. Ardashir I founded the dynasty, while Shapur II and Khosrow I were earlier rulers, not successors to Yazdegerd.