Shapur II was the tenth Sasanian King of Kings of Iran. The longest-reigning monarch in Iranian history, he reigned for the entirety of his 70-year life, from 309 to 379.

His reign saw the military resurgence of the country, and the expansion of its territory, which marked the start of the first Sasanian golden era. He is thus along with Shapur I, Kavad I and Khosrow I, regarded as one of the most illustrious Sasanian kings. His three direct successors, on the other hand, were less successful. At the age of 16, he launched enormously successful military campaigns against Arab insurrections and tribes who knew him as 'Dhū'l-Aktāf ("he who pierces shoulders").

Shapur II pursued a harsh religious policy. Under his reign, the collection of the Avesta, the sacred texts of Zoroastrianism, was completed, heresy and apostasy were punished, and Christians were persecuted. The latter was a reaction against the Christianization of the Roman Empire by Constantine the Great. Shapur II, like Shapur I, was amicable towards Jews, who lived in relative freedom and gained many advantages in his period. At the time of Shapur's death, the Sasanian Empire was stronger than ever, with its enemies to the east pacified and Armenia under Sasanian control.