Shapur I (also spelled Shabuhr I; Middle Persian: 𐭱𐭧𐭯𐭥𐭧𐭥𐭩, romanized: Šābuhr) was the second Sasanian King of Kings of Iran. The precise dating of his reign is disputed, but it is generally agreed that he ruled from 240 to 270, with his father Ardashir I as co-regent until the death of the latter in 242. During his co-regency, he helped his father with the conquest and destruction of the city of Hatra, whose fall was facilitated, according to Islamic tradition, by the actions of his future wife al-Nadirah. Shapur also consolidated and expanded the empire of Ardashir I, waged war against the Roman Empire, and seized its cities of Nisibis and Carrhae while he was advancing as far as Roman Syria. Although he was defeated at the Battle of Resaena in 243 by Roman emperor Gordian III (r. 238–244), the following year he was able to win the Battle of Misiche and force the new Roman emperor Philip the Arab (r. 244–249) to sign a favorable peace treaty that was regarded by the Romans as "a most shameful treaty".

Shapur later took advantage of the political turmoil within the Roman Empire by undertaking a second expedition against it in 252/3–256, sacking the cities of Antioch and Dura-Europos. In 260, during his third campaign, he defeated and captured the Roman emperor, Valerian. He did not seem interested in permanently occupying the Roman provinces, choosing instead to resort to plundering and pillaging, gaining vast amounts of riches. The captives of Antioch, for example, were allocated to the newly reconstructed city of Gundeshapur, later famous as a center of scholarship. In the 260s, subordinates of Shapur suffered setbacks against Odaenathus, the king of Palmyra. According to Shapur's inscription at Hajiabad, he still remained active at the court in his later years, participating in archery. He died of illness in Bishapur, most likely in May 270.

Shapur was the first Iranian monarch to use the title of "King of Kings of Iranians and non-Iranians"; beforehand the royal titulary had been "King of Kings of Iranians". He had adopted the title due to the influx of Roman citizens whom he had deported during his campaigns. However, it was first under his son and successor Hormizd I, that the title became regularized. Shapur had new Zoroastrian fire temples constructed, incorporated new elements into the faith from Greek and Indian sources, and conducted an extensive program of rebuilding and refounding of cities.

Shapur was a popular name in Sasanian Iran, being used by three Sasanian monarchs and other notables during and after the Sasanian era. Derived from Old Iranian *xšayaθiya.puθra 'son of a king', it must initially have been a title, which became—at least in the late 2nd century AD—a personal name. It appears in the list of Arsacid kings in some Arabic-Persian sources; however, this is anachronistic. Shapur is transliterated in other languages as; Greek Sapur, Sabour and Sapuris; Latin Sapores and Sapor; Arabic Sābur and Šābur; New Persian Šāpur, Šāhpur, Šahfur.

According to the semi-legendary Kar-Namag i Ardashir i Pabagan, a Middle Persian biography of Ardashir I, the daughter of the Parthian king Artabanus IV, Zijanak, attempted to poison her husband Ardashir. Discovering her intentions, Ardashir ordered her to be executed. Finding out about her pregnancy, the mobads (priests) were against it. Nevertheless, Ardashir still demanded her execution, which led the mobads to conceal her and her son Shapur for seven years, until the latter was identified by Ardashir, who chose to adopt him based on his virtuous traits. This type of narrative is repeated in Iranian historiography. According to 5th-century BCE Greek historian Herodotus, the Median king Astyages wanted to have his grandson Cyrus killed because he believed that he would one day overthrow him. A similar narrative is also found in the story of the mythological Iranian king Kay Khosrow. According to the modern historian Michael Bonner, this story of Shapur's birth "may conceal a marriage between Ardashir and an Arsacid princess or perhaps merely a noble lady connected with the Parthian aristocracy." According to A. Shapur Shahbazi, this is one of several stories meant to legitimize Sasanian role by connecting Ardashir and his successors with the Parthian nobility. In the tale, the birth of Shapur to Zijanak takes place after the overthrow of Artabanus, but C. E. Bosworth states that Shapur I must have been born "well before" this event (i.e., before 224); the Sasanian rock reliefs and the history of al-Tabari indicate that Shapur took part in Ardashir's battles against the Parthians. In his inscriptions, Shapur identifies his mother as a certain Murrod.

Shapur I was a son of Ardashir I and his wife Murrod The background of the Sasanian family is obscure; although based in Pars (also known as Persis), they were not native to the area, and were seemingly originally from the east. The historian Marek Jan Olbrycht has suggested that the family was descended from the Indo-Parthians of Sakastan. Iranologist Khodadad Rezakhani also noted similarities between the early Sasanians and the Indo-Parthians, such as their coinage. Yet, he stated that "evidence might still be too inconclusive."

Pars, a region in the southwestern Iranian plateau, was the homeland of the southwestern branch of the Iranian peoples, the Persians. It was also the birthplace of the first Iranian Empire, the Achaemenids. The region served as the center of the empire until its conquest by the Macedonian king Alexander the Great (r. 336–323 BCE). Since the end of the 3rd or the beginning of the 2nd century BCE, Pars was ruled by local dynasts subject to the Hellenistic Seleucid Empire. These dynasts held the ancient Persian title of frataraka ("leader, governor, forerunner"), which is also attested in the Achaemenid-era. Later under the frataraka Wadfradad II (fl. 138 BCE) was made a vassal of the Iranian Parthian (Arsacid) Empire. The frataraka were shortly afterwards replaced by the Kings of Persis, most likely at the accession of the Arsacid monarch Phraates II (r. 132–127 BCE). Unlike the fratarakas, the Kings of Persis used the title of shah ("king"), and laid foundations to a new dynasty, which may be labelled the Darayanids.

Under Vologases V (r. 191–208), the Parthian Empire was in decline, due to wars with the Romans, civil wars and regional revolts. The Roman emperor Septimius Severus (r. 193–211) had invaded the Parthian domains in 196, and two years later did the same, this time sacking the Parthian capital of Ctesiphon. At the same time, revolts occurred in Media and Persis. The Iranologist Touraj Daryaee argues that the reign of Vologases V was "the turning point in Parthian history, in that the dynasty lost much of its prestige." The kings of Persis were now unable to depend on their weakened Parthian overlords. Indeed, in 205/6, Pabag rebelled and overthrew the Bazrangid ruler of Persis, Gochihr, taking Istakhr for himself. Around 208 Vologases VI succeeded his father Vologases V as king of the Arsacid Empire. He ruled as the uncontested king from 208 to 213, but afterwards fell into a dynastic struggle with his brother Artabanus IV, who by 216 was in control of most of the empire, even being acknowledged as the supreme ruler by the Roman Empire. Artabanus IV soon clashed with the Roman emperor Caracalla, whose forces he managed to contain at Nisibis in 217.