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Asoristan (Middle Persian: 𐭠𐭮𐭥𐭥𐭮𐭲𐭭 Asōristān, Āsūristān), also known as Suristan, was the name of the Sasanian province of Assyria and Babylonia from 226 to 637.

The Parthian name Asōristān (𐭀𐭎𐭅𐭓𐭎𐭕𐭍; also spelled Asoristan, Asuristan, Asurestan, Assuristan) is known from Shapur I's inscription on the Ka'ba-ye Zartosht, and from the inscription of Narseh at Paikuli. The region was also called several other names, mostly relating to its indigenous Assyrian inhabitants: Assyria, Athura, Bēṯ Nahren (Classical Syriac: ܒܝܬ ܢܗܪܝܢ lit. 'between the two rivers'), Bābēl/Bābil, and Ereḫ/Erāq. After the mid-6th century, it was also called Khwārwarān in New Persian. Although it is difficult to determine the true meaning of the renaming of southern Mesopotamia to "Assyria", it is possible that due to the Assyrian kingdom of Adiabene being in the north, it excluded the Assyrians further south and therefore the Persians named it accordingly to include the few million Semitic Mesopotamian people who were descendants of the ancient Assyrians south of Adiabene, contrary to the Greeks renaming Assyrians "Syrians", an Indo-European corruption of "Assyrians". At the very least, names associated with Assyria continued to be used and were not obsolete.

The name Asōristān is a compound of Asōr "Assyria" and the Iranian suffix -stan "land of". The name Assyria, in the form Asōristān, was shifted to include what had been ancient Babylonia by the Parthians, and this continued under the Sasanians, the Babylonians being ethnolinguistically the same people as the Assyrians. During the Parthian Empire, much of historical Athura lay to the north of Asoristan in the Assyrian inhabited independent frontier provinces of Upper Mesopotamia (in modern Northern Iraq, Southeast Turkey and Northeast Syria): Beth Nuhadra, Beth Garmai, Adiabene, Osroene, Hatra, Tyareh and Assur. when the Sassanid Empire conquered these in the mid-3rd century, they were reincorporated into Asoristan.

During the Achaemenid (550–330 BCE) and Parthian Empires (150 BCE – 225 CE), Achaemenid Assyria had been known by the Old Persian name Athura. Asōristān, Middle Persian "land of Assyria", was the capital province of the Sasanian Empire and was called Dil-ī Ērānshahr, meaning "Heart of Iran". The city of Ctesiphon served as the capital of both the Parthian and Sasanian Empire, and was for some time the largest city in the world.

The main language spoken by the Assyrian people was Eastern Aramaic, which still survives among the Assyrians, with the local Syriac language becoming an important vehicle for Syriac Christianity. The Assyrian Church of the East was founded in Asōristān and it was an important centre of the Syriac Orthodox Church.

Asōristān was largely identical with ancient Mesopotamia. The northern border is somewhat uncertain but probably went along a line from Anta^[where?] to Takrīt. Ḥīra was probably the southernmost point, north of Arabia, the border then following the northern part of the swamps of Wasit.

The Parthians had exercised only loose control at times, allowing for a number of Assyrian kingdoms to flourish in Upper Mesopotamia in the form of independent Osroene, Adiabene, Beth Nuhadra, Beth Garmai and the Arab-Assyrian state of Hatra. Assyriologists such as Georges Roux and Simo Parpola opine that ancient Assur itself may have been independent during this time.