Shapur I, son of Ardashir I, was king of Persia from 241 to 272.

The Persian legend which makes him the son of an Arsacid princess is not historical. Ardashir I had towards the end of his reign renewed the war against Rome; Shapur conquered the Mesopotamian fortresses Nisibis and Carrhae and advanced into Syria; but he was driven back by Timesitheus , father-in-law of the young emperor, Gordianus III, and defeated at Resaena in 243.

Shortly afterwards Timesitheus died, and Gordianus was murdered by Philip the Arab, who concluded an ignominious peace with the Persians (244). When the invasion of the Goths and the continuous elevation of new emperors after the death of Decius (251) brought the Roman empire to utter dissolution, Shapur resumed his attacks.

He conquered Armenia, invaded Syria, and plundered Antioch. At last the emperor Valerian marched against him, but Valerian was taken prisoner in Roman-controlled province of Edessa, when he attempted to meet for negotiations (260). Shapur advanced into Asia Minor, but was beaten by Ballista; and now Septimius Odenathus, prince of Palmyra, rose in his rear, defeated the Persian army, reconquered Carrhae and Nisibis, captured the royal harem, and twice invested Ctesiphon (263 - 265).

Shapur was unable to resume the offensive; he even lost Armenia again. But according to Persian traditions, which appear to be trustworthy, he conquered the great fortress of Hatra in the Mesopotamian desert; and the great glory of his reign was that he kept a Roman emperor prisoner to the day of his death. In the valley of Istakhr (near Persepolis), under the tombs of the Achaemenids at Naksh-i Rustam, Shapur is represented on horseback, in the royal armour, with the crown on his head; before him kneels Valerian, in Roman dress, asking for grace. The same scene is represented on the rocks near the ruins of the towns Darabjird and Shapur in Persia. Shapur left other reliefs and rock inscriptions; one, at Nakshi-Rajab near Persepolis, is accompanied by a Greek translation; here he calls himself "the Mazdayasnian (worshipper of Ahuramazda), the god Sapores, king of kings of the Aryans [Iranians] and non-Aryans, of divine descent, son of the Mazdayasnian, the god Artaxares, king of kings of the Aryans, grandson of the god-king Papak." Another long inscription at Hajjiabad (Istakhr) mentions the king's exploits in archery in the presence of his nobles.

From his titles we learn that Shapur I claimed the sovereignty over the whole Earth, although in reality his domain extended little farther than that of Ardashir I. Shapur built the great town Gundishapur near the old Achaemenian capital Susa, and increased the fertility of this rich district by a barrage through the Karun river near Shushter , which was built by the Roman prisoners and is still called Band-i-Kaisar, "the mole of the Caesar." He is also responsible for building of the city of Bishapur, which was built by Roman soldiers who had been captured after the defeat of the Roman emperor Valerian in 260. Under his reign the prophet Mani, the founder of Manichaeism began his preaching in Persia, and the king himself seems to have favoured his ideas.